Wider is always better (unless you're sensitive to the increased harshness of the lower-profile sidewalls of the wider tire, something I doubt anyone putting a V-8 in a Volvo would complain about).

The two limits are tire compatibility and clearance.

The first is merely determined by what width wheel you're using: 6 or 7 inches or more? The 205/55 can take a 6 inch wheel, but the 225 may be a little wide for that wheel width (it may be within the limit of compatibility, but the sidewalls are going to be canted inward, and that's not good for cornering well). The 225/50 deserves a wider wheel, at least.

The issue of clearance depends on the size wheels and their offset -- these will determine where the tire (and its sidewall) will be positioned 'in space' relative to the hardware in the wheel well.
For example, on my 240, I'm using Hydra wheels which are 6.5x16 with a 25 mm offset (standard for 240s is a 20mm offset). I use 205/55 tires*, and these wheels bring my inboard sidewalls pretty close to the suspension pieces in both front and rear wells -- as close as 1/4 to 1/2 inch, stationary, and that's as close as I'd like to get, because while the car is cornering hard (and when I can't measure, for obvious reasons), I expect that the sidewalls are shifted to the side slightly. So I'm glad I didn't go out and buy a set of 225/50 tires instead, even though I was tempted because of the additional tread width, because the extra 0.5 inch inward shift of the sidewalls would rub.
However, if my wheels were different (i.e., different offset), or if I used spacers, then I could imagine that the 225/50 width could fit.
So consider not only the wheel size, but also it's offset, in planning where the tire's inboard sidewall will be positioned.

Good luck.

[ * By the way, the tires I use, in my example about my sidewalls' clearance, are Michelin Pilot Exalto PE2's. I don't think other tires would be much different, though.]