Yup, it's better to have a Klein- or C-lator with D-style amps. If you use effects in the loop like reverb or delay it's a must with most units.
I don't think the sound difference is THAT big with the c-lator (this is what I use). You can search for a thread I made where I recorded the difference with and without the C-lator.
Also the bright switches are very cool
My advice: If you use effects in the loop, get one. If not, it's a cool extra feature but not really necessary.
For me, the C-lator took my Lexicon LXP-1 to a whole new level