I assume you mean effects, not preamps and things?
Depends, if you use a lot of effects, then MIDI-controlled rackmount is the way to go, since you can program a patch that has, say, 4 effects you need for your lead tone, and another with 5 other effects for the rhythm tone. Also, if the rest of your righ is rack-based, getting rackmount fx unit is a logical step.
If all you use is just a tad of delay and reverb (which you can get from the amp anyway), maybe just getting one or two pedals would be simpler and cheaper...
I use the effects in my preamp, but might upgrade to a rackmount fx-unit one day...