I would certainly clarify the terms of your Letter of Appointment before accepting it, particularly in relation to the meaning of "provisional". Current Army policy states that an Aviation Cadetship is an avenue for entry into the Army whereby the applicant has successfully completed the testing required and is pre-selected to Army Aviation. There is no mention of a Cadetship being provisional, however the Defence Personnel Regulations and subsequent Defence policy does permit applicants to be provisionally appointed.
The terms contained within your letter appear to not preselect you to Army Aviation. A provisional Cadetship therefore essentially means nothing - you will be competing for Aviation in the same way every other graduate competes for their desired Corps. Even upon successful completion of RMC, if there are not sufficient positions within the Corps you will be assigned elsewhere based off Service needs. Your only avenue back into Aviation would be to commence a Corps transfer. Lord knows when, and if, you would be succesful. This is a substantial risk that you would accept and something you must carefully consider. It is also unlikely that you will become aware of posting positions until quite late in your RMC training, making resigning impossible until you have served out your ROSO for the RMC component.
This risk cannot be underestimated. I have personally seen it occur and it is devastating to the individuals concerned. The most likely scenario for your situation is that several of your possible classmates are on a normal Cadetship. You have been offered a provisional Cadetship and will be selected to Aviation in the event that one of them fails, is back classed, resigns, is injured etc. If none of the above occurs you may not have a position to be posted to.
If Army Aviation is your thing, I highly recommend the GSO stream. Everyone will give you their two cents worth but the simple fact is that you have substantially more career choices later on. And yes, staying in a flying position is indeed one of these. I would however, strongly advise you to clarify why your Cadetship is provisional and what you would have to do to be offered an unconditional Cadetship. Get absolutely everything in writing and then keep copies of it all in the most safe place possible.
Finally, out of curiosity, what is the ROSO/IMPS stated in the letter?