Both machines are completely different and incompatible, as with MK6 to Gen7 (despite some Gen7 games looking exactly like MK6 games but in a newer cabinet). The LAB comms board (assuming NSW) can be used in both MK5 and MK6 machines however. Belly panel artwork/paytables can fit into both MK5/MVP and MK6 (Xcite) cabinets, although the Xcite glass is slightly thinner so it can bend easier around the curved Xcite belly panel. It is also possible to convert a MK5 series 1 cabinet to MK6.
The MK5's CPU was an old ARM250 and the MK6 had a Hitachi SH4. In other words, it's like comparing a an
Acorn A3000 computer to a
Sega Dreamcast. Worlds apart.
MK5's EPROMs were 27C4096 types (4mbit/512K) while MK6 EPROMs were 27V322 (32mbit/4MB).
If you have a MK5, hold onto it as there are a number of games which were never ported to later machines, which might be of interest if you played some of the older games e.g. Cash Chameleon was only ever on MK5. Adonis is fairly common though, being that it's just another Queen of the Nile/Dolphin Treasure clone, and it was also on MK6 under a number of names e.g. Adonis = Torch of the Gods = Flame of Olympus.