The move steer and move tank blocks take 5kb each while the large motor block takes 4kb. If you have a long string of move blocks in your program, it will take up big chunks of memory. We've gotten around that in the past with using myblocks. Myblocks are only copied to the NXT once and called each time it is needed in the program. A pivot myblock might take 7kb but you can use it as many times as you like without taking up more memory. We make a myblock for just about every common move used in our programs. It pays to make a myblock if a string of code will be used more than once.
I would stick with EV3 software if you plan to move to the EV3 hardware in the next season or two. I personally liked NXT-g better but we upgraded our hardware to EV3 this year.
Edit - I just compared the NXT blocks to the EV3 blocks and there is quite a difference. The NXT-g move block only takes 2.4KB and the motor block takes 1.1KB. Our pivot myblock that takes 7KB in EV3 only takes 1.6KB in NXT.