I believe most of us use the Eheim/Jager heaters with very few issues. IMO, all heaters may have issues simply by virtue of what they do, but it seems like the Eheim/Jager heaters have the fewest issues.
As far as size goes, I'd probably use two 200w heaters, one set slightly higher than the other one so it only comes on if the first one can't keep up on the coldest days.