The hydor theo 25watt and 50watt are exactly the same size and fit perfectly in the pump chamber. I cut a small hole in the outflow line so that the water in the pump chamber would circulate better and be deep enough for the heater.
The 50 watt on the absolute lowest setting keeps my tank at 80 degrees.
I have an Elite Mini 25w in my Spec V and when it gets cold it has a hard time keeping the temperature at a constant level usually it drops by 4-6 degrees. I would go with a 50w if I was going to buy another one down the road.
The Aqueon 50w does indeed look snug. Based on its dimensions I thought it was going to be too tall when I looked at it. Still not sure if I want the heater in direct or near-direct contact with the pump and outflow line.
I had the Aqueon Elite mini heater in a 5-gal a while back, but after about 6 months something went wrong with it, raising the tank temp to 90, cooking all my fish and shrimp I'm hesitant to try it again.