Most cabinet manufacturers make upper section taller. (some will have an option) With custom cabinetry, you can have it equal or any which way you like...

The reason they make lower portion taller is that lower portion is always used more, so in many cases, lower portion will have a rollout shelving and an upper portion where people put stuff away which is hardly used or used on the occasions, is why on top they use stationary shelving.

It pretty much comes down to personal preference because you and only you will be using the cabinets so you want convenience or if you want the symmetric look, you keep it equal.

Are you actually dividing the cabinets vertically, or just the doors? Because I think you'd just want tall cabinets with whatever combination of pull-outs and shelves works for you, and doors as is convenient. If there is some other functional separation, like one of the bottom sections is your trash, then separate cabinets makes more sense.

As for which parts you use most, I'd say whatever is at eye level -- all our best stuff is between waist and chin height, where it's easy to see and to access things without a stool or leaning over. So I think the longer doors on top make more sense in that regard, as both doors and contents will be in the most convenient zone. But if you will have racks of stuff that you want to see from above (not always the best labelled part of packages), then maybe taller lower doors would make sense...

acm, generally, where ever the door is split, there's a fixed shelf, creating two separate spaces, top and bottom. They do this to add stability to it so it won't rack.

Thanks for all the comments. Yes, I will go with bottom portion to be larger. Now I just need to figure out the right proportion. Thinking if 96" (i know there is a kick&molding....so probably 90" workable), I will go about 72" on bottom and 24" on top.