oscar is just stating the given rule for wattage per gallon of water, a 300 watt is bit over kill for a 55, if the cost difference between a 200w and 300w is a large range.. then it makes sense to get the smaller one. buying bigger is usually a waste, since it costs more to operate it, vs a smaller unit. (same thing applies to a gas furnace, if you get an oversized unit vs sq feet of the house, it will cost more to fire up the burners and keep it running, than it would it was properly sized to the house)
i have used thermal compact eclusively, over the past 10yrs and they are very realible and cheap heaters.. [i agree with you] i do like them alot however that's only because the LFS
stock those, visitherms and ebo jager and the therms and jagers are quite a bit more expensive. and it's usually a difference in spending 20bucks vs 30bucks on jager.
only reason i am looking to change from the thermal compacts is, online all the heaters are all pretty much the same price, and i have to order 3 misc parts for other things.. (costs 6bucks and 6 to ship) so i might as well buy online.