First of all welcome to WMExperts!!!!!
The first question I would ask is what network do you want to use? 8525 is GSM with Cingular and the Mogul is CDMA with Sprint. The difference can be drastic with coverage geographically. The data speed is faster and a whole lot cheaper on Sprint. If for whatever reason Sprint of Cingular is knocked out of a reasonable possibility due to coverage issues, data speeds, or pricing for talk time and/or data.....then no need to compare the two phones further as it will not matter much.
If both carriers are a viable options then even though the Mogul and the 825 are direct competitors, generally speaking, it is probably better to think of the 8525 as comparable to the PPC-6700 and the Mogul as the next generation of the 6700......though I am sure this statement could cause a reasonable debate.
Some of the upgraded features for the Mogul when comparing it to the 8525 are that the Mugul has WM6, Dual processor (one processor for the phone and the other for the PDA side), jog wheel, double the ROM (same RAM), smaller, etc....
When comparing the phones, you will want to see and use each one in hand. Go down and play with the demo phones of each.
The bottom line I give to anyone who is looking for a new phone is to buy the phone that meets YOUR needs and wants. Just because the Mogul is the most perfect fit for me does not mean it may be the best phone for you. List the Pros for each phone (coverage, features, layout, etc...). Then list all the cons (i.e. plan pricing, lacking feature, known issues with the phone, etc..). If the pros meet you needs, and then you can around, or use a 3rd party solution to fix the cons and live with the others that cannot be fixed......then you have the perfect phone for your professional needs and your personal wants and you should be very happy with whatever phone you buy.
I hope this helps.