Well they suggest about 2.5 watts per gallon depending where you ask. So a 45 gallon would need 112.5 watts of heater. Other places say 2-5 watts per gallon.
The closer to 2 WPG, the more gently and slowly it will heat the water to set temp compared to if you use 5 WPG it will reach set temp faster and more abrupt in comparison. I'm sure it'd be a minimal difference to be noticed, but never the less. I myself have a 75 watt on my 29g which is exactly 2.5 if you call it a 30g instead of 29g. I have it attached to a ReefKeeperRKL1 and it has NEVER dropped below 77.5 where it is set to come on and set to turn off at 77.7
Everytime I check, it is always within those parameters. Note I also have a series of 6 fans (one plug) plugged into the RKL that turn on at 80 degrees to cool it down.
And that's with a crumby used no-name glass heater that was like $15 new back in the day. So imagine titanium.
WonBrothers tends to get on the lower side of reviews TBH and 300 watts does sound like overkill to me.
Jager is still good as noted, but if you want to go another route, ViaAqua Titanium heaters are good as well and seem to get good ratings and come in both 100 watt and 150 watt. I'd go 150 if I were you. But their controllers are built in external and look pretty poorly made.
IMHO i'd go Jager.