The 3/4 upper is our most popular seller.
Here's my 2¢: The 3/4 allows you to reach the back of the bottom grate a little easier, so if you cook smaller items on the bottom grate, and some larger things up top then it's a great option. HOWEVER... if you're going to be cooking periodically large family gatherings, etc. Go with the FULL upper rack. Trust me on this! Based on 8+ years of cooking on a MAK, that full space is handy for those cooks!
As far as needing to reach the rear of the bottom grate, normally when I need the upper rack I have something large on the bottom, so reaching the bottom rear has rarely ever been an issue.
In fact I'm getting ready to order a second full upper rack for those times when I load the MAK up with chicken for graduation parties, etc. It will make swapping the top & bottom racks half-way through the cook so much easier!
...when smoking I never use the bottom grates. I use a foil pan for drippings.
Smoke, as you continue using your MAK, you may find that changes some as you experiment with different items in different combos.
Some examples:
Chicken/poultry always cooks better on the bottom, hands down (for me at least). The skin just crisps up much better.
Other things such as ribs, pork shoulders, brisket, I've done them both ways numerous times and honestly cannot say that one has worked better than the other, so for us it depends on other circumstances. If I want to catch the drippings, then I tend to go for the top rack with a pan underneath. If I'm adding side dishes then the protein is usually on the bottom rack, which leaves room for adding other things up top later.
Baked goods biscuits, cookies, pies, etc. (higher baking temps); the top rack definitely wins.
Alright so it was more like 4¢ and probably still not worth 2¢!