both are very similar so it depends if you prefer the laptop form or the tablet form.
both can use a pen. but Surface Pro has better sensitivity and latency. at this point, it's unclear if Surface Laptop has the same digitizer and hardware because if it did, they probably would've touted the improved pen for that. Microsoft has stated that tilt, better sensitivity, and better latency of the pen only works on the new Surface Pro and not on older Surface Pro machines (though tilt may be added in a future firmware). but since the Surface Laptop is a laptop that doesn't fold all the way back, using a pen may be awkward.
both have almost the same features and specs.
The Surface Pro does have a MicroSD slot for expanded storage. I put a 128 GB card in my Surface Pro 4. I don't think the Surface Laptop has a MicroSD slot.
I'm a Surface Pro user all the way. The most intriguing aspect for the Surface Laptop for me is the colored metal and being able to open it one-handed.