Two questions to ask yourself:
Does the size of the yard justify a wider deck? A 54" deck will significantly cut your "time in the saddle" if your lawn is more than 1/2-3/4 acre. The larger deck won't save you as much time on a smaller yard, and you may also have tight spots it won't fit through.
Do you have a lot of trees, and hence, leaves to pick up in the fall? If so, the Power Flow is definitely worth what it costs. It's also nice to have if you like to bag your grass clippings, but mulching blades are a lot cheaper.
Both have the diff lock, PS, lift, liquid cooling, hydraulic lift, so are pretty equivalent. Time on both tractors is essentially the same, so again, pretty equal. Snowblower will fit both, equal there, too.
Brush guard is pretty, but does little in terms of actual work, and my guess is that you don't get into areas where it would do what it's designed to do very often, so it doesn't factor in for comparison of value.
So.... the biggest difference in cost of the equipment is the Power Flow, because both GT's were available with either 48" or 54" decks for about the same.
If your need for the Power Flow is valid, go with the smaller deck and more expensive tractor, but I think the better value is the GX345 with the larger deck for $1900 less. You can buy a Power Flow for much less than that.