Which should I buy.
For riding a mixture of off road stuff.

I've got the 853 & love it. Coming from a very stiff GT Alu frame it made a noticeable difference.

Guess you need to decide what's important, weight or comfort. Approx 1 lb difference between them….

hhhmmm, good point re the weight.

I got an 853 for my better half. Its great-light, comfy, fun!

So the 853 seems to be most popular at the moment.

853 wins hands down over ally ok it's heavier but the ride will be better and they are cheap as chips for 853

The aluminium Tig Team SL is made from Easton Ultralite tubingand weighs around 3.25 lbs. The 853 is about 4.5 lbs, quite light for a steel frame. In terms of ride quality, the 853 offers a slightly more compliant ride feel due to the smaller diameter tubes, the Tig Team SL will feel a little stiffer/more responsive.

If you call/email Merlin I'm sure they'll be happy to help, plus they have a shop where you can try bikes.

Cheers
I spoke to Merlin this morning and they were very helpful.
I wanted the opinion of the STW massive to avoid a fashion faux pas.