There are a number of questions that you ask here, lets get through them one by one:
1) Capco salary offer - Do not take an MC job to work for less money than you believe you can get elsewhere. MC are hard environments where you work long hours and some weekends, with lots of time away from home. The last thing you want is to be doing that while earning less... There is ALWAYS room for negotiation but you also need to be realistic about your earning potential. If RBS and Capco are offering you less than your expectations, are you really worth what you think!?! Nonetheless, I would negotiate with Capco and take it from there
2) RBS means you are joining "industry". There are lots and lots of threads on this forum on "industry vs MC". Do a search and have a read, it will tell you what the guys here think and its not rocket science re workloads, travel required etc
3) Ignoring RBS offer - I strongly suggest you do not, why close doors so early? Instead, if you are unsure ask them if you could have a coffee with a Change Manager from the department to have a chat about the place you will be joining. Be honest, say you have heard things about RBS but you are still excited about the role, hence would like to learn more before diving in. You have an offer remember, they want you. Have a look and take it from there. Negotiation wise, there is a little bit of wiggle room, you may be able to argue for a bit more but not much
Have fun, its always nice to have a choice and you will be going into 2 very different career routes whichever you choose. But remember, going to an MC allows you to join a bank later, similar to the way that joining a bank also allows you to go into MC later.
Lastly - be honest with yourself on the type of person you are. I cannot emphasise this one enough. If you are not hard nosed, able to do lots of brown nosing and comfortable with Partners level bullpoop, you will be stuck at SC level in any MC (inc Capco) for a very long time. In industry your ability to deliver will play a greater focus on your promotions. Hence do choose carefully.
Coolio, Marsday and BEP will also have some good insights for you.