There are many different types and brands of effects and processors out there, but choosing which one that is right for you is not only a matter of taste but also what type of amp you have. Whether you play bass or guitar, the best way to decide is to bring your guitar or bass and amp with you to the music store. But if not, then make sure they have the same gear in stock as you are using because you don't want to find out later that it doesn't sound the same.
If you plan on running your sound through a mixer or powered P.A. speaker, then you want to go with an effect processor that has a built in preamp. The reason for this is that the distortion effect will sound too harsh when ran through a P.A. mixer or into a recorder. The price of effect processors with a built in preamp have come down in price compared to when they first came out.
One effect processor with a built in preamp is the Vox ToneLab ST. It has an actual 12AX7 tube to drive its power. The built in preamp section allows the unit to be ran through a P.A. mixer or recorder. The built in effects (flange, chorus, tremolo, phase, and delay) sound great. The distortion/overdrive, amp and speaker cabinet simulators equally sound great. The built in expression pedal can be assigned to be used as either a volume control or a wah pedal. The expression pedal can also be assigned as a control pedal to change the rate settings used for the phase, flange, and other modulated effects. This unit also has a USB interface to easily manage and edit all of your presets on your computer. The Vox ToneLab ST is very compact, making it easy to transport.