I have 2 x 250w stainless steel heaters in the sump for my 220g.
The reason I went with 2 x 250's is because I could get them really cheap at the time, the temp in this tank is not critical and I will be keeping it at about 22 celsius, seeing that the tank is in my bedroom I'm hoping it wont require too much from the heaters. At the moment the weather here is so warm that if anything we have problems cooling the tank, I'm waiting for winter to see if they will be ok, I'm expecting they might struggle