depends on a couple of factors. ask yourself these questions about what you want (or may want soon)...
how long do you want/need loops to be?
Do you want to be able to save loops?
ability to have loop decay? Adjustable decay rates?
Do you want to be able to erase a layer that you don't like without erasing the first four that you did (undo function)?
real estate (size) the pedal takes up...
internal midi clock (allows for perfect synching to other midi synchable devices)
Boss RC-2 - If you are just going to practice in your room, then this is is a great starting place for looping. It has a long record time (about 3 minutes depending on available memory) for one track, so you can practice over changes, or a whole song. Also has auto reverse and the ability to store loops (a rarity for the price). one shot feature (repeats single phrase once every time you hit it). Not too expensive.
But it does have some major draw backs- no undo, i.e. mess up, start over, not very gig friendly. the actual tracking isn't great IMO. My DL4 (what I use now) is spot on in comparison. this won't matter when you are playing by yourself (you won't even notice because it is slight) but I found it impossible to use live with a drummer/or drum machine in real time. If you adjust the tempo of a stored loop it shifts the tone bigtime.
Line6 DL4- You can instantly reverse and also drop an octave (half time). This feature is REAL fun if you like to experiment. i.e. loop a drone tone, then put into half time mode, record a simple high register motif, then take it out of half time. result = the original looped texture is normal register/octave/speed while the motif is played double time (and higher octave). go back and forth like this a couple times, maybe reverse the whole thing and voila! it sounds like something you could only coax out of a laptop! You can also add a bit of echo and or modulation on the signal that is being looped.
It also has the ablity to slowly decay when left in overdub mode, which is nice for certain applications. One shot feature on this as well.
big additonal plus- it comes chalked full of all kinds of modeled delays w/tap tempo (tape, 24bit, modulated, etc...) also cool reverse lead, auto volume swell mode (very cool) and more. Note, when in loop mode, you cannot access all of these sounds (only the modulated echo). I wish I had two because of this!
Downsides, short loop time (30 secs or so). Doesn't bother me cause i use it for shorter rhythmic loops and textures. can't store anything, power down and its gone. no undo function. It is supposedly "true bypass" but some folks still find it sucks some tone. Put it in your effects loop if you have one.
Still for the price and creative inspiration, when using live, I found it far superior to the Boss, which i have and never use anymore.
Boomerang- if you have the space and cash, I would go for this. Multiple levels of undo. multiple decay rates. Long record time. auto- reverse, drop, octave, ability to adjust playback volume with foot (huge plus for live playing). the new one due soon is supposed to be even better (better sound quality, more features, daisy chain multiple units, etc..) yet it has a smaller footprint. Mike neilson says it will drop any minute. This is my next gear purchase.
Down sides- no saving loops. Larger foot print. still no midi clock (only available in top tier devices). also $100 more than those mentioned above.
haven't tried the EH 2880, but hear good things.
Premium tier- Gibson/Oberheim echo-plex digital pro, Electrix repeater (discontinuted but one of the best ever). Too many features to list. check the websites. not cheap.
The mac daddy- Looperlative
Whatever you get, I am sure that it will spur some fresh ideas, so take the plunge!
two cool links for lots more info:
http://www.loopers-delight.com/loop.html - for all the info
http://livelooping.tribe.net/ - online community of looper enthusiasts
Good luck!
