I just left Qinetiq a few months back, so here's my 2p...
There are lots of full time QQ FTEs at Boscombe, mostly focussed on release to service. These guys are one of the big drivers behind the QQ-MoD Long Term Partnering agreement - basically so that MoD gets independant advice about aircraft safety.
There are a number of other people in QQ who are qualified FTEs but serve in more research oriented roles, so they're full time employees but not full time FTEs. I worked with both sorts & dabbled in the research flying myself - as did many of the rest of my team (mostly sim bods).
ETPS is run by Qinetiq, so the link is extremely close - QQ will put the appropriate people through the FTE course if it needs them. I believe there was normally a bit of a delay whilst folks served as trials officers before the need arose for a formal FTE qualification.
Now, in the 7 years between the split & me leaving I never met a DSTL equivalent of the former - MoD trusts QQ (and BAES & AW...) to do stuff like that.
I did meet DSTL types that supported research flying and went out into the field (sandy places) as tactics & technology advisors.
So really, it's up to you. My impression would be that there are more jobs going at QQ & they're more likely to hire a trials officer out of Uni & stream them for FTE work. DSTL, if you can get in, is likely to be more fun - but you will have to live near Portsdown West on the South coast eventually.
And of course, I bow to those still south of the M4...