Hi,
This exact thing happened to me last year - in fact, I posted about it on this site as I was looking for similar advice! In the end, I turned down the studentship I was offered as I knew deep down it wasn't for me and I would be very unhappy doing it for 3 years. A few weeks later I was lucky enough to then apply for and be granted an RA job with the same institute (but a different location) which allowed me to gain some experience in that area. During this period, a separate funding bid I had put in was successfully funded so I'm starting my ideal PhD in September.
So from my experience, it all worked out fine in the end - but at the time, turning down the initial studentship was a huge gamble as I had no idea whether these other opportunities would manifest. I guess I'd have to say to go with your gut feeling (whatever that may be) like I did and hope that whatever you choose pays off!