I have the Equitrek Space Treka (2016 model) and I blimmin love it! It tows well, is plenty big enough for my 16.2 beast plus a friend, has the really useful tackroom at the front, and all of the horses and ponies travel well in it. It can get warm in the summer as all trailers can, but if you open all of the windows a decent breeze sorts that out. I've had it over 2 years now (have had it serviced regularly). We're out and about every weekend at pony club or meeting friends to hack, and i've not had any serious issues except a sticky jockey wheel (thread easily realigned and sorted) and a flat tyre (yet... touch wood...hope I haven't jinxed things!). I have no experience of the Cheval trailer though, so can't comment on that one. I know equitrek do get bad reviews, but I must admit I love mine!
ETA - Just realised that the ST III is stalled for 3 and rear-loading. Mine is stalled for 2 and is side-loading. Sorry! Ignore the above if not relevant!