I have a 550 in my 1959 K35. I do not regret going to the 550. My airplane originally had a IO470C. I upgraded to a 520 about 20 years ago. About 5 or six years ago, I found a used 550 and upgraded to that engine.
I do not fly my airplane faster with the 550 than I flew it with the 520. Although it really will go faster, IF I WANTED to. The main advantage of the 550 is:
1. Shorter take off. Really nice when departing high altitude airports.
2. Better rate of climb. With the 470, it was a long slow climb to 9 or 10 thousand feet altitude. With the 520, the climb was quicker to 10 K. With the 550, I have no reluctance to cruise climb to 12000. This is nice when dealing with my trips to the west coast.
3. Speed. The Bonanza is a fast airplane. However, I usually cruise at 21 inches/ 2300 RPM. Or FT (full throttle). I usually cruise at 11.5 - 12.5 GPH. My TAS is usually 155 - 160. I have tip tanks. So I have at least 7.5 - 8.0 hours of fuel at my cruise power. This means that I can fly 1000 Nautical miles non stop, if I want to, AND still have 1.5 hours reserve. I rarely fly more than six hour legs.
IF I were to fly at high cruise speed, I would have to stop every four hours. At 155 KTAS (180 MPH), I can get 15 MPG.
If you don't need to fly high, then the 520 is an excellent engine. However, the 550 is a much nicer engine. In my humble opinion.
Economic wise, it is a difficult choice. Please note that I sold my 520 for 2000 less than I paid for my 550. I already had the STC for the 550 when I upgraded to the 520, so that was not a cost issue. You need the 3 blade prop with the 550 (not an option). Still, the three blade prop is nice on a Vtail as it moved my CG forward 1/2 inch and improved my ground clearance.
So, the choice is up to you. You pays your money. Just don't expect to have your airframe increase in a value equal to the upgrade.
Again, I do not regret my decision.