Also, airplanes cover a LOT more ground a LOT faster than helis do, so a site that is plenty big for a 90 size glow chopper can be way too small for even a small size airplane.

You're doing forward flight with the heli right? Any basic acrobatics? If so, then you won't have a problem. If you're just hovering, and can't do even a few moments nose-in, then you might have some issues with an airplane. A plane is easier, but it is also different. How much differnet and how "easy" depends more in the person than on any one thing.

(I mostly fly planes, but I do have a heli, and I've flown a couple helis here and there).

If you want to fly on a soccer field, keep it electric and keep it on the smaller, slower end of things, no bigger than speed 400 size. That would rule out the big telemaster, but the mini telemaster would be a possible choice. It's still a bit big, but it flys slowly and could be kept in such a small area.

3 soccer fields is SMALL by airplane standards. Really small. You're talking ~120 yards by about 120 yards, right? The runway at my club is longer than a soccer field, and about as wide, and that's just the runway, not including the area beyond each end used to approach and depart over, and never mind all the open space out away from the runway that is overly-fly area.