i dunno if this makes any difference but here's what i set up like 3 weeks ago and its made a huge difference in my 70G that isn't drilled...
i was totally upset tho for awhile cuz my tank conditions were sucking before i set this up. you can read a couple stories online, anyways, here goes:
i got an an adjustable overflow box coming out of my DT, its adjustable cuz there are screws that lift or lower the overflow. this raising/lowering somehow (magically) makes the flow into my refugium go faster or slower! totally amazing. this kinda acts like a surface skimmer too (another person mentioned that already). this is what creates the siphon to my below the tank refugium set up.
the flow back up, i use a rio. simple. i added this weird pump thing that came with the overflow box, its this thing that pulls the water up (and any air bubbles) in case of a power outage, to start the siphon again once the power comes on. its attached to a lil hole on top of the siphoning box.
i was originally going to put the fuge above my tank (decided against it once i figured the lil shelf would break and i'd have hell on earth). but the cool thing was i totally figured out that even tho i bought all this stuff to set it up, all i had to do was switch around the equipment, taking the overflow out of the fuge and puttin it in the DT and the rio out of the DT and into the fuge!
anyway, the decision is yours. i think the benefit, like someone else said, depends on what you want out of a fuge. if i grew like feeder critters in my fuge and think they're gonna end up in my DT, it would be better to have above tank fuge setup. but like having chopped up pieces of pods up through a rio is fine by me for my below tank setup.
i have live sand in there right now and some live rock crumbs and like the spaghetti looking macro algae. the LFS dude was a sweetie and like gave all of this for free! then i bought a "daylight" bulb and clippy thingie with reflectors from home depot. oh yeah, and i got a 15$ mangrove tree! i found the tank in the garbage! no leaks!
i've only had it set for like 3 weeks now and there is so much lifecoming out of my DT! the water quality is amazing! tiny little snails are popping out at night, like 50-60! a peppermint shrimp that has been hiding (thought he was dead) for 4 months has come out. and my green hair algae is disappearing. the cyano-bacteria problem is gone. totally gone!
i was gonna like pack it in and kick the bucket on reefing and the refugium has absolutely saved me! i'm a believer baby. i think ever newbie like me should be told to set up a refugium from the start. i don't have a sump, or a deep sand bed, just a crummy coralife super skimmer and an eheim cannister. i'm like so ghetto but seriously, don't give up on your 75G, fuge it! anyways, good luck with yer 29-20 setup. by the way, don't bother with the two powerhead idea, its like so impossible to get them in sync. oh and i'd go with cheato, why bother with caulerpa, i dont even get what going sexual means but cheato's fine. i've already pruned my once, it like loves that home depot light. the mangrove is kinda pretty too. eventually it'll get too big (its a tree baby!) but its pretty now.
oh yeah one more thing. if you wanna look at it is also another thing to consider and that took me like awhile to decide (i'm a girl, whatcha wanna say bout it!). if you don't care how it looks and you're not planning on "displaying" it, then just get one of those plastic storage containers. some peeps believe a fuge is like a filtering swamp, equating it to like ecological principles. i decided it would be fun to set it up so i could view it at night (opposite light schedule to stabilize ph). having decided that biased my decisions from an aesthetic point of view. so i recognize that alot of the decisions which would be more beneficial for utility, i had to overide. i like sand but 5 inches for a DSB is too much.
anyways, thats what i went thru. i know people don't like me posting here but i hope that kinda helped.
oh yeah, last thing, my skimmer is HOB off my DT. i read somewhere that if you skim in your fuge, you'll just be wasting skimmering before your macro and mangrove get the nutrients. my skimmer is barely picking anything up in my main tank compared to pre-fuge days.
oh, and my tank runs pretty warm already and i found that putting the heater in my fuge heats up the fuge too much. and my live rock and live sand are in the main part of the fuge with my macro, the mangrove is in the corner a lil ways from da light so i don't burn the leaves.
- xoxompty
... so some say that having sand in a refugium doesn't make a difference. however, does it make a difference if you have a BB display? 
