Most of our "normal" walking is on gravel/stone paths with a dog in tow and so we put a high priority on good suspension, runflat tyres and a hand brake. We opted for the Hartan GT Racer - it has been superb in delivering what we wanted it to do - effortless over rough terrain. The downside is that it is a fairly bulky piece of kit - and the X3 won't take it without unclipping the base. With the bassinet still in it takes up a substantial amount of space. For journeys with luggage we have to tether the base to the headrest in a vertical manner - which is a faff.
Build quality is excellent and Mini-Me has had no problems with it.
At Christmas we will be picking up an Oyster Switch in the UK for travelling - as it does parent facing - the only similar one we found over here was the Quinny Zapp and that that felt rather flimsy - especially when a piece of plastic broke off one while the shop assistant and I were folding it!!
Next year we'll also be getting a jogger/biking/x-Country-skiing trailer. Croozer or Thule is the current debate on that front