I would opt for two half racks. You may want an additional full rack sometime. Two half racks would give you that as well as the option of using only one half rack.
1) If you have the $$ up front, I'd get both a 1/2 and a full upper grate so you have maximum flexibility.
2) If you are only going to buy one, then the full rack will let you take advantage of the full capacity if needed. You could always add the 1/2 rack later when budget allows.
Of course the 3/4 size will split the difference quite nicely, and you could still add another size later on as budget allows.
I would opt for two half racks. You may want an additional full rack sometime. Two half racks would give you that as well as the option of using only one half rack.
For pre-2015 2 Stars I agree wholeheartedly! It's how I went with my 1 Star. BUT...
The 2015+ MAK 2 Stars (and the 2016+ 1 Stars) use a different rack system which has no legs. They slide into a channel that supports them. I don't think the channel is deep enough to support two half racks.
Cliff, you're correct, the newer style set up won't support the two half racks.
Mds2004, the three quarter rack has certainly been the most popular shelf size-- it gives you a good amount of extra room up top, but still allows you to reach the food in the bottom back of the grill (which is trickier with the full rack). Hope that helps!