LakeWake... I agree to go with the bigger sack, you do not have to fill it all the way if you do not want to.
Check out this thread "questions on PROPer setup"
viewtopic.php?f=15&t=1563&start=10I did a side by side comparison of the FH 750's versus the SL Sumo 600's. You can decide for yourself what is better, but if by "new" you are talking about getting a 2013, then you will have the 1" inside diameter hoses, fittings, and bigger pumps... go with Straightline's SUMO series for sure, they are all 1" I.D. and as you will see from my write up, they are, in my opinion, better sacs and the 600's appear to be the same weight as the 750's.
Also, if you can wait until April to order your sacs, you can get the SUMO axis bow sack that is better and bigger than the Fly high bow sack. It has the 1" ID fill and drains, and it is expected to be $50-$100 cheaper than the fly high sack. In the same proper set up thread I referred to earlier is my write up on the sumo bow sack and pictures.