Wow, tough question. I have both and here are my thoughts.
Dreamverb - Unlimited ability to create a room ambiance for instruments or to add just a touch of liveness to some tracks. Vocal uses: not my first choice as it cannot do the silky smooth vocal sound required on say a ballad. The Plate 140 is much better for this. DV is a great tool for shaping a space that may not exist when recording at home or in a bad room.
MX200 - This has great smoothness and authenticity as a high end reverb, just not as many quality presets but is equal to the ambiance task as Dreamverb if not better sounding overall. It has the added ability to provide very good but bright vocal ambiance vs the Plate 140 which is darker. Then there is the VST implementation which is good and bad. Good because its nice to run an external box like a plug in, bad because your computer software cycles through all the settings during updates and files saves, slows the computer down temporarily.
I have the the M-One from TC Electronics as well which covers a lot of the Plate 140 in an external box. Deep rich ambiance.
Basically they are all different colors which get applied in various places. The most universal box would be the M-One if I could only have one. This would be followed by the Plate 140 because it kind of defines the sound of vintage reverb. Next would be the Lex MX200 followed by Dreambverb, which is a precision problem solving tool. Nice to have but not meat and potatoes like the rest.
Hope that helps.