The Mordaunt Shorts will probably give you a fuller sound, but not quite the detail on offer from the KEF's - I think it'll depend on the type of music you listen to...
Personally, I'd go for the Kef's.
I've currently got a AV setup based on the Onyko 605 with Bose Acoustimas 5 series III in front and a pair of Kef 2005.2 eggs as rears. I'm looking for a pair of relatively inexpensive but reasonable quality discounted or S/H floorstanders (hence the previous model Mordaunt Short or Kef) that will do me to replace the Bose until I've put enough £££ aside for a 5.1 system which I was thinking about building based on a pair of the Mordaunt Short Mezzo 6's.
So once I have the 'temporary' floorstanders I'll also want to buy suitable S/H centre and subwoofer, which I will upgrade when I've moved up to the Mezzo 6's. The advice I've been given is don't mix makes for the front and centres but choose the floorstanders that I plan to end up with and plan for those.
Also the Kef iQ5se look like going for £395 new and around £200-250 used. The Avant 916i are £299 new and ??? used.
I realise that this seems like a laborious way to go about building a 5.1 system, but having just moved house I can't really afford to splashout on £1,750 (or thereabouts) all in one go.