If you read the reviews on the submersible UV units most people say they are worthless. Problems with leaks into the housing are the biggest complaint. Having the tear the whole thing out of the tank to do any maint. is another complaint. IMO there's too much that can go wrong from simple operator error.
I found a 8w HOB style from Aqua UV for my 30g. Purchased an Eheim 1048 pump, 6' of 1/2" tubing and 6" of 3/4" flexi-tube (to put on the outside of the 1/2" where it crosses the rim). I fashioned a pre-filter out of some dual-stage pool filter sponges I picked up at Lowe's. This system was pricey (for a 30g), but with my set up I can move it to any of my tanks should an emergency arise.
One thing you should look into is the price of the replacement bulbs. Some brands have odd prongs or odd wattages, thus forcing you to buy only their bulbs and precluding you from buying discount versions.
