I would go with as large a refugium as I could fit under the stand.Just dont go so tall with it ,so harvesting algae will not be too hard.And I would use a Mag18 or maybe a large LittleGiant MDQ5.There are many advantages to the refugium over the trickle filter.If you have a reef tank it is going to feed the corals and fish a fresh supply of creatures that would normally be ate before they get to a large number.Even if it is a fish only tank,you will be able to grow algaes in the refugium to feed the fish in the display tank as well as have them for reducing Phosphates and nitrates in a natural form.(not that it will keep them to zero,it couldnt be that easy in this hobby)hehe.I dont mean to make it such a long post ,but I hate to see people spend good money twice.(and Im a LFS manager ,believe it or not)hehe.I deal with people daily that are switching over from T/F to refugium with great success.
Good Luck
reeferguy
Do not consider a tricle filter. Use live rock and sand and a refugium.
Go very slow with the additions of livestock in your tank and avoid the problem types and you will be sucessful.
I just set up a new 300 6 months ago. I use a Dolphin AmpMaster 3000. It's a very quiet cool running pump. It delivers good return from the sump. I also have a Iwaki MD70 as a general circulation pump also to add more water flow. It runs on a closed loop from the tank. As for a trickle/sump, I had to have it built custom because I wanted at least a 80 gal sump. I could not find one that large as a standard size. Mine was designed as a trickle and bought it w/o the balls. It can hold 40 gallons of balls if I decide to use it someday for a fish only tank. That would rate it somewhere around 500 to 700 gal tank cap.