What are the applications for the sub?...how big is your room?
I sell and install the Rel's and M&K's both. I'd have to say that, both companies make a great product. I think, when both subs are set up properly(and even EQ'd from the listening possition for absolute best results), in ultimate terms, the Rel's might have a smidgen more low end detail, pace, and musical refinement...but just a smidgen. The M&K has an edge in sheer brute extension and authority I think, and is right close with the Rel in terms of speed and quickness (close). Both are musical fast subs, that integrate with a large variety of speakers. The M&K's build quality and longevity are first rate for sure! ...haven't sold the Rel's long enough (or done enough volume). I more lean towards the M&K MX5000 a bit more for my applications however, as I'm into home theater applications with my subs, but also do high end 2 channel with em some times.
Again, probably the biggest benefit you'll get, like alway's, no matter wich sub you chose, would be to get the sub set-up properly (which most, even seasoned audiophiles, don't do correctly)!! In order to get truely satisfying, tight, flat response, non peaky or sucked out, great dynamic range from the sub you need to have the right knowledge and some patience.
I'd say, given the number of high end subs I've sold over the years, to people who usually end up installing em themselves,most people never ever get anywhere near what a sub is cappable of in their systems!...they think it's either just acceptable, or as good as it gets!...or so they think. 99% of the time, there's an easy 200% or greater potential out of the sub/bass they can get, making any small performance differnces between any competetive subwoofer virtually insignificant in the big picture...this is no understatement!
So, guess what I'm saying is, if you're only into music, I'd probably look into hearing both subs (Consider taht they'll likely not every be set-up right when you hear em unfortunately, even at competent audio shops!). The Rel is as good, tight, detailed and refined as most you'll ever come across for music in their classes. The M&K is right close, and can get tremendous performance with proper care (like the Rel). However, if you're doing double dubty for music/movies', I'd alway's go with the MX5000 in most every case! They're bulletproof, hard hitting, dynamic, and fast as is ever needed for that application...and go deep for a large room!
For really large rooms though, 2 is best, at least.
Remember, set-up is still 2/3's of you're battle for great sound from any bass driver! Bass is the single most challenging thing to get right in most every normal house/room setting, and the thing most people never get even close to doing right! Get some professional guidance at this price level for sure, or learn how to do it, if you don't already...best to you, and have fun.