rfc - there's a lot to think about here ...
I don't have a direct answer for you, but I do have a lot of good points for you to ponder, so you can make the best decision that fits your specific situation.
The difference in deck size comes down to time. You mention that you're interested in starting a lawn business. Just how big? Is it something you'll do full time, or as a second job? Are you the only person mowing, or will there be others? Etc etc. Deck size will determin how much time you save or spend. However, don't get greedy at the expense of your wallet and succumb to the whole American philosophy of "If something is good, then more must be better". Too big has issues as well. Do you have gates, trees, a/c units, etc that you have to get in between? Only you know these anwers; we can't tell you what only you know. You can, with diligence, sit down and figure out how much time you'll save. It comes down to how many passes are cut for any acre you mow. At any given speed, you'll make "X" passes. Do the math on your projected lawn load, and see where it falls. For example, if you consider all other items equal (cut overlap, travel speed, etc), then a 61" deck will save you about 3.5 minutes for every acre you mow, over a 52" deck. If you only mow less than 10 acres a week, that's not really very much time you'd save. If you mow 15 acres a day, every day of the week, it can add up. This is why commercial crews use big rigs; it is cumulative. But if you're only doing this as a part time, extra pocket cash gig, then the added expense probably isn't worth it. And don't forget about physical constraints of size, including your trailer!
The FX engine is a more robust engine for true commercial use. These are meant to run 8-10 hours a day, every day of the week. If this is how you'll use it, then the FX is a great choice. If you intend to run this lawn business on the side, to supplement your income, and you'll only mow a few extra hours a week, then the FS would likley do well.
How much power you really "need' (not want, but need) is a function of how much load you expect to pull. If you expect very hilly areas, and heavy wet grass, etc, then get more power. If smooth, level lawns are presesnt, and you'll only mow in the afternoon after the dew is done, then you can get away with a little less power.
One last thing; consider used equipment if this will be a part-time gig. You can save a bundle on someone else's asset depreciation. Don't buy a clapped-out piece of junk, but there are some decent used units out there, if you look hard enough.
My point is that we cannot tell you what to buy, but we can give you good info on how to make the selection that makes you most happy with your decision. The more info you use, the better and more refined your decision will be.
All food for your thoughts.