@Eddie LeBlanc : The Markbass is probably one of the quietest comps out there. Even the Keeley can't compare in that regard. Other than that, the 2 are very different beasts. The MB is a tube comp and the Keeley is a VCA comp. If you are liking the sound of the MB, you may not like the Keeley. Tube compression has a characteristic 'softer' and 'colored' tone that is added to the signal, while many VCA comps (certainly the Keeley) are rather transparent and possibly a bit faster reacting without having to over-compensate on the compression ratio. I prefer the dbx compression sound, and the Keeleys give me that in a pedal. What I didn't like about the MB was the high-current 12v power it needs. Besides that, the MB is most likely just about as good as you can get with a tube compressor, rack or pedal, bar none! Well played!
If you are looking to get into a VCA compressor, the Keeley Bassist sounds as good as the Pro, but without the separate Attack and Release or the 7 led meter, and is about $100 USD less! It has an led that lights when the signal is past threshold which is, really, all the information I need to set it properly. I also find the Bassist attack and release is always just the way I would set them if I were using a rack or digital mixer comp. Keeley says "Attack and Release times vary on program content". What I don't want is to be chasing settings during a show, so this works for me.

Best of luck!