Sorry for the double post
[TABLE="class: vs_s_table ie_wfix, width: 100%"]
[TR]
[TD="class: title, width: 32%, bgcolor: #C0C0C0, align: center"]Intel Core i5-4200U[/TD]
[TD="class: title, width: 32%, bgcolor: #C0C0C0, align: center"]Intel Core i5-4200Y[/TD]
[/TR]
[TR="class: row1 vs_sm"]
[TH]Market segment[/TH]
[TD="colspan: 2, align: center"]Mobile[/TD]
[/TR]
[TR="class: row2 vs_sm, bgcolor: #E8E8E8"]
[TH]Manufacturer[/TH]
[TD="colspan: 2, align: center"]Intel[/TD]
[/TR]
[TR="class: row1 vs_sm"]
[TH]Family[/TH]
[TD="colspan: 2, align: center"]
Core i5 Mobile[/TD]
[/TR]
[TR]
[TD="class: vs_spec_h, colspan: 3"]Basic details[/TD]
[/TR]
[TR]
[TD="class: vs_spec_sep"][/TD]
[TD="colspan: 2"][/TD]
[/TR]
[TR="class: row1h, bgcolor: #D8D8FF"]
[TH]Model number[/TH]
[TD]i5-4200U[/TD]
[TD]i5-4200Y[/TD]
[/TR]
[TR="class: row2h, bgcolor: #C0C0E8"]
[TH]CPU part number[/TH]
[TD]CL8064701477702[/TD]
[TD]CL8064701557900[/TD]
[/TR]
[TR="class: row1 vs_sm"]
[TH]Introduction date[/TH]
[TD="colspan: 2, align: center"]June 2, 2013[/TD]
[/TR]
[TR]
[TD="colspan: 3"][/TD]
[/TR]
[TR]
[TD="class: vs_spec_h, colspan: 3"]CPU features[/TD]
[/TR]
[TR]
[TD="class: vs_spec_sep"][/TD]
[TD="colspan: 2"][/TD]
[/TR]
[TR="class: row1 vs_sm"]
[TH]Core name[/TH]
[TD="colspan: 2, align: center"]Haswell[/TD]
[/TR]
[TR="class: row2 vs_sm, bgcolor: #E8E8E8"]
[TH]Platform name[/TH]
[TD="colspan: 2, align: center"]Shark Bay[/TD]
[/TR]
[TR="class: row1 vs_sm"]
[TH]Microarchitecture[/TH]
[TD="colspan: 2, align: center"]Haswell[/TD]
[/TR]
[TR="class: row2 vs_sm, bgcolor: #E8E8E8"]
[TH]Technology (micron)[/TH]
[TD="colspan: 2, align: center"]0.022[/TD]
[/TR]
[TR="class: row1 vs_sm"]
[TH]Data width (bits)[/TH]
[TD="colspan: 2, align: center"]64[/TD]
[/TR]
[TR="class: row2 vs_sm, bgcolor: #E8E8E8"]
[TH]Socket[/TH]
[TD="colspan: 2, align: center"]BGA1168[/TD]
[/TR]
[TR="class: row1h, bgcolor: #D8D8FF"]
[TH]Frequency (MHz)[/TH]
[TD="class: vs_b"]1600[/TD]
[TD="class: vs_w"]1400[/TD]
[/TR]
[TR="class: row2h, bgcolor: #C0C0E8"]
[TH]Turbo Frequency (MHz)[/TH]
[TD="class: vs_b"]2600[/TD]
[TD="class: vs_w"]1900[/TD]
[/TR]
[TR="class: row1h, bgcolor: #D8D8FF"]
[TH]Clock Multiplier[/TH]
[TD]16[/TD]
[TD]14[/TD]
[/TR]
[TR="class: row2 vs_sm, bgcolor: #E8E8E8"]
[TH]L1 cache[/TH]
[TD="colspan: 2, align: center"]64 KB (code) / 64 KB (data)[/TD]
[/TR]
[TR="class: row1 vs_sm"]
[TH]L2 cache (KB)[/TH]
[TD="colspan: 2, align: center"]512[/TD]
[/TR]
[TR="class: row2 vs_sm, bgcolor: #E8E8E8"]
[TH]L3 cache (KB)[/TH]
[TD="colspan: 2, align: center"]3072[/TD]
[/TR]
[TR="class: row1 vs_sm"]
[TH]Max temperature (?C)[/TH]
[TD="colspan: 2, align: center"]100[/TD]
[/TR]
[TR="class: row2h, bgcolor: #C0C0E8"]
[TH]TDP (Watt)[/TH]
[TD="class: vs_w"]15[/TD]
[TD="class: vs_b"]11.5[/TD]
[/TR]
[TR="class: row1 vs_sm"]
[TH]Cores[/TH]
[TD="colspan: 2, align: center"]2[/TD]
[/TR]
[TR="class: row2 vs_sm, bgcolor: #E8E8E8"]
[TH]Threads[/TH]
[TD="colspan: 2, align: center"]4[/TD]
[/TR]
[TR="class: row1 vs_sm"]
[TH]Multiprocessing[/TH]
[TD="colspan: 2, align: center"]1[/TD]
[/TR]
[TR]
[TD="colspan: 3"][/TD]
[/TR]
[TR]
[TD="class: vs_spec_h, colspan: 3"]Instruction set extensions[/TD]
[/TR]
[TR]
[TD="class: vs_spec_sep"][/TD]
[TD="colspan: 2"][/TD]
[/TR]
[TR="class: row1 vs_sp"]
[TH]AES / Advanced Encryption Standard[/TH]
[TD="colspan: 2, align: center"]+[/TD]
[/TR]
[TR="class: row2 vs_sp, bgcolor: #E8E8E8"]
[TH]AMD64 / EM64T 64-bit technology[/TH]
[TD="colspan: 2, align: center"]+[/TD]
[/TR]
[TR="class: row1 vs_sp"]
[TH]AVX / Advanced Vector Extensions[/TH]
[TD="colspan: 2, align: center"]+[/TD]
[/TR]
[TR="class: row2 vs_sp, bgcolor: #E8E8E8"]
[TH]AVX2 / Advanced Vector Extensions 2[/TH]
[TD="colspan: 2, align: center"]+[/TD]
[/TR]
[TR="class: row1 vs_sp"]
[TH]FMA3 / 3-operand Fused Multiply-Add[/TH]
[TD="colspan: 2, align: center"]+[/TD]
[/TR]
[TR="class: row2 vs_sp, bgcolor: #E8E8E8"]
[TH]MMX[/TH]
[TD="colspan: 2, align: center"]+[/TD]
[/TR]
[TR="class: row1 vs_sp"]
[TH]SSE[/TH]
[TD="colspan: 2, align: center"]+[/TD]
[/TR]
[TR="class: row2 vs_sp, bgcolor: #E8E8E8"]
[TH]SSE2[/TH]
[TD="colspan: 2, align: center"]+[/TD]
[/TR]
[TR="class: row1 vs_sp"]
[TH]SSE3[/TH]
[TD="colspan: 2, align: center"]+[/TD]
[/TR]
[TR="class: row2 vs_sp, bgcolor: #E8E8E8"]
[TH]SSE4.1[/TH]
[TD="colspan: 2, align: center"]+[/TD]
[/TR]
[TR="class: row1 vs_sp"]
[TH]SSE4.2[/TH]
[TD="colspan: 2, align: center"]+[/TD]
[/TR]
[TR="class: row2 vs_sp, bgcolor: #E8E8E8"]
[TH]SSSE3 / Supplemental SSE3[/TH]
[TD="colspan: 2, align: center"]+[/TD]
[/TR]
[TR]
[TD="colspan: 3"][/TD]
[/TR]
[TR]
[TD="class: vs_spec_h, colspan: 3"]Supported technologies[/TD]
[/TR]
[TR]
[TD="class: vs_spec_sep"][/TD]
[TD="colspan: 2"][/TD]
[/TR]
[TR="class: row1 vs_sp"]
[TH]Hyper-Threading[/TH]
[TD="colspan: 2, align: center"]+[/TD]
[/TR]
[TR="class: row2 vs_sp, bgcolor: #E8E8E8"]
[TH]PowerNow! / Enhanced SpeedStep[/TH]
[TD="colspan: 2, align: center"]+[/TD]
[/TR]
[TR="class: row1 vs_sp"]
[TH]Turbo Core / Turbo Boost[/TH]
[TD="colspan: 2, align: center"]+[/TD]
[/TR]
[TR="class: row2 vs_sp, bgcolor: #E8E8E8"]
[TH]Virtualization[/TH]
[TD="colspan: 2, align: center"]+[/TD]
[/TR]
[TR="class: row1 vs_sp"]
[TH]Virus Protection / Execute Disable bit[/TH]
[TD="colspan: 2, align: center"]+[/TD]
[/TR]
[TR]
[TD="colspan: 3"][/TD]
[/TR]
[TR]
[TD="class: vs_spec_h, colspan: 3"]Integrated Graphics[/TD]
[/TR]
[TR]
[TD="class: vs_spec_sep"][/TD]
[TD="colspan: 2"][/TD]
[/TR]
[TR="class: row1h, bgcolor: #D8D8FF"]
[TH]GPU Type[/TH]
[TD]HD 4400[/TD]
[TD]HD 4200[/TD]
[/TR]
[TR="class: row2 vs_sm, bgcolor: #E8E8E8"]
[TH]Base frequency (MHz)[/TH]
[TD="colspan: 2, align: center"]200[/TD]
[/TR]
[TR="class: row1h, bgcolor: #D8D8FF"]
[TH]Maximum frequency (MHz)[/TH]
[TD="class: vs_b"]1000[/TD]
[TD="class: vs_w"]850[/TD]
[/TR]
[TR]
[TD="colspan: 3"][/TD]
[/TR]
[TR]
[TD="class: vs_spec_h, colspan: 3"]Integrated Memory Controller(s)[/TD]
[/TR]
[TR]
[TD="class: vs_spec_sep"][/TD]
[TD="colspan: 2"][/TD]
[/TR]
[TR="class: row1 vs_sm"]
[TH]The number of controllers[/TH]
[TD="colspan: 2, align: center"]1[/TD]
[/TR]
[TR="class: row2 vs_sm, bgcolor: #E8E8E8"]
[TH]Memory channels[/TH]
[TD="colspan: 2, align: center"]2[/TD]
[/TR]
[TR="class: row1 vs_sm"]
[TH]Supported memory[/TH]
[TD="colspan: 2, align: center"]DDR3L-1333, DDR3L-1600, LPDDR3-1333, LPDDR3-1600[/TD]
[/TR]
[/TABLE]
As you can see, the major difference between the Y variant and the U variant is the frequency, and it isn't even something that the average person would notice as the CPU isn't being taxed on anything other than gaming. Even then, a good GPU will be more important.