Generally speaking,
Monotubes:
good = lasts a long time (significantly longer than twin tubes), better heat dissipation (less chance of overheating & failing), less chance of foaming (gas-pressurized), quicker reaction & recovery to bumps, better handling/braking (due to stiffer ride)
bad = stiff ride, can get damage easier from debris/rocks hitting the shock because it is "one tube", costly to replace
Twin-tubes:
good = cheaper, slightly tougher due to "one tube within another tube" design, softer (comfier) ride,
bad = does not lasts as long as monotubes, not as efficient as dissipating heat, can overheat and fail with prolong continuous use, not as quick reacting, softer ride
Remember, these are generalizations. However, from my experience, i found that these generalizations for the most part hold true. Within each class, there are slight differences in the shock characteristics from brand to brand due to valving and other stuff. For example, top of the line KYB and Bilsteins are both monotubes, but KYB is stiffer than Bilsteins (from what i hear). Or take Tokico aftermarket shocks...i think that they are twin-tubes, but are stiffer than stock OEM shocks (twin tubes also).
XREAS shocks are monotubes by the way from the factory.