The F210Z is basically the same mower as the SnapperPro S200xt. My father has an S200xt with a suspension seat. It is an excellent, high end commercial mower, and the suspension seat did greatly improve the ride. The IS700Z is a mid level commercial mower. It too is a very good mower, but the drive train on it isn't as good as what is offered on the F210Z. The suspension on the IS700Z doesn't have as much travel as the IS2100Z models and up, so the ride on the IS700Z isn't as smooth. The ride on the IS700Z is about the same as the ride of a rigid frame mower with a suspension seat.
Either mower would be fine for your lawn. Both come with the same ICD deck. The suspension seat offered on the F210Z makes the suspension on the IS700Z a wash. The Vanguard offered on the F210Z is a higher end commercial engine than the Commercial Turf on the IS700Z. The ZT5400 hydros on the F210Z is heavy duty commercial, where as the ZT3400 hydros on the IS700Z are mid level commercial. The F210Z would be overkill for 3.5 acres, but it is the better mower of the two. In truth, the IS700Z could easily handle your lawn for many years, as long you keep it properly maintained. It just depends on wether you want to pay the extra $2k for a top of the line drive train.
I mow my 4.2 acre lawn, and my grandmother's 4 acre lawn with a 48" SnapperPro S150XT. It has the ZT5400 hydros, and a high end commercial FX series Kawasaki engine. The ICD deck leaves a great cut. My grandmother has a pond on her property with very steep banks. There are only two small areas that I can't mow with my S150XT. My dad can mow those two areas with his S200XT without problem. I hope the information I have given helps you with your decision. Please let us know which mower you chose, and why.