Been to the careers office? If not go down there, get some more info. My recruiter gave me a really rubbish computer 'game', more and more literature and a funny look when I told him I was interested in being a gunner. Helps to talk to someone who really knows and not wannabes like me.
It is (was 30 years ago) perfectly possible to transfer after you have time in. I had the seven best years of my life in armoured recce (now called formation recce) before transferring to RAPC (now AGC) and training on IBM mainframe computers. I now work for the second biggest corporation in the world and don't regret much of it at all.
For chrissake don't make a decision about what you want to do with your whole life based on what might happen down the line. Make the appropriate choice now (which is obviously RAC) then worry about what you are going to do next further down the line.
In my day, the day-to-day of the RA was hell compared with the day-to-day of RAC. If you make the wrong decision now, it'll screw you for years to come and potentially turn your head the wrong way.
If you want to play with boy-toy armoured vehicles every day, join the RAC. If you want to be bullsh!tted from arrsehole to breakfast time 50 weeks a year then fire a few rounds of large-calibre ammo the remaining two, join the RA.
(That's how it was in the 1970s: don't blame me if it's all changed in the last 30 years.)