Every subject has its up sides and its down sides and I think you are very wise to be considering the subject you want to end up teaching before you do your degree. Is it possible for you to get experience in some schools? Maybe just some observation of different subjects and how they teach?
You will find that different subjects have different methodologies and I think it would be worthwhile considering this as part of your decison making process. For example, RE teaching (which I do!) involves a lot of class discussions. I know some teachers who love it, and others who hate the whole holding debates thing (usually scientists lol!). Would you like to do that? If not, RE would not be a wise choice!
The benefits of teaching RE are that the lessons are often very interactive, you have slightly more freedom than in other subjects (as there is no national curriculum) and you only (usually) have to put with students for an hour a week (unless its an exam class), so if its a nightmare class, you don't have them for too long! However, the downsides are that RE is often not taken seriously by students or parents. Often well meaning but ill informed parents try to dismiss the subject when they haven't really got one iota of what the subject is about and they make judgements on the subject based on what they did 30 years ago - and the subject has undergone a lot of change in just the last 10 years I have been teaching it, let alone the last 30! This can rub off on students and you sometimes hear them saying 'why do we have to do this, my mum says its a waste of time?' and things like that, which can get a bit draining after a while.
But there are downsides to other subjects too - for example, I remember discussing the pros and cons of RE / English teaching with a friend, and her biggest grievance was that if she had a difficult class, then she saw them 4 times a week, and that could be very hard going, particularly if they didn't like quiet reading!
See if you can talk to teachers, or even observe! There are times when I wish that I had thought more about what subject I wanted to teach when I was at uni - I just wanted to teach and didn't care what, now 10 years down the line, I do somewhat regret that.
Finally, if you do decide to go in for RE teaching, then don't do an RE degree. I would say that either a theology or a philosophy degree would be more respected if you wanted to take on 'A' level teaching. Certainly, when I have been involved in the interviewing process in the past, Theology degrees have been more preferable as they are considered to be more academic.
HTH