I always like to go with 5 watts per gallon as a "rule of thumb." It varies depending on your geographic location and your system. Systems with sumps generally need more heat than systems with HOB filtration, systems with skimmers can require more due to the air contact with the water, etc.
I've never deviated from 5 watts-per-gallon no matter what type of system I have and it's always served me well.
I also subscribe to the 2 heater methodology aptly described by rigdon. So in your case, since you have a 55g system and one 300w, rather than replace it, I'd add another 300w and just have a little more than you need. You're not ridiculously over, so it'll just mean they don't have to run as long.