Hi Christine, oh gosh, like 2 years late, but I just came across this so I thought I'd respond. You probably know all this now...
Siterone is not the same as Oestrogel. Siterone is cyproterone, an anti-androgen, Oestrogel is 17-Beta (human) estradiol. One is not a substitute for the other. It's my belief that you should not take an anti-androgen without also taking oestrogen. To have neither hormone in the system is dangerous, expecially long term, and will lead to osteoporosis and other problems.
As to your question, yes Oestrogel is undoubtedly safer than Siterone, and the DVT risk from Oestrogel is extremely low. 17-Beta estradiol is the safest form, much safer than synthetics or equine oestrogen. And when used in gel form as opposed to tablets it is safer still. In fact it's probably the safest way possible to take oestrogen. Patches are also good, but slightly more impractical I think, (plus they give me a big square rash), and injections are considered a safe way of taking oestrogen as long as you know what you are doing with needles. I didn't fancy that, and gender clinics in this country tend to go for other routes of delivery. The gel is pretty much foolproof, I imagine you could smother your whole body in the stuff and still not overdose (though I do NOT recommend trying). It comes in a pump dispenser giving a small, measured dose each time. It's quick, easy, leaves no residue and has no smell. It takes seconds to "disappear". 
And it works. I love the stuff.
xx