The truth is MUSH clients are probably very damn close to the cutting, bleeding edge of what they can potentially do since they are limited by the telnet protocol's limitations. So unless MU* codebases begin offering more properietary features clients can take advantage of (which some have, but none have become even remotely a standard, and I've yet to see something I'd consider an actual great feature worth switching for) a program last updated in 2001 and one whose newest version came out today will have few distinctive advantages.
I paid for SimpleMU* back in the day but I'd be happy to ship out a few $$ again for something I use every day, it's just that other clients either don't have as-you-type spell checking (which I find very handy) or spawns (which I find absolutely essential). So that ol' program actually superior to stuff that will just let me look at the same text but with more shades of orange or red, which to me is an irrelevance and could actually be counter-productive based on the grid's builders.
Others milage may vary but this is mine.