I'll jump in here with the other side, even though I think the advice above is great.
If you're concerned about your budget getting started, the AD-20 certainly costs less, especially if you're using mics that are internally powered. Remember that you'll have to buy/build a power supply for the UA-5 as well, which can level the cost of phantom power.
For that matter, I love my AD-20. It really cleans up the low end, which is great with most "starter" mics like the C1000s and the MK012s. The Denecke PS-2 is a great choice for phantom power, but it's $150. Rolls makes a unit called that PB224 that's $100 new, but I got one for $50 (shipped!) off their website as a "second" -- it was missing a rubber foot. (I actually took the rest of them off to make it easier to fit in my case.)
These units both sound good, and both have some good points and bad points. Neither of them have a good way to monitor levels (the UA-5 has peak lights, the AD-20 has nothing), but you can learn to use the meters on the NJB3. Factor in the cost of a battery pack on the UA-5 or phantom on the AD-20. The UA-5 does have a better upgrade path.
Have fun. Check out some shows with each before you make a decision!
Chuck