How to politely decline an offer

Hi, my dilemma is the following:

I joined a commodities trading firm (oil & gas) just three months ago in Switzerland. Had to move from London, which was quite a big move. The salary is really good, the job is challenging and seems to be interesting so far (I'm a trading assistant, fresh graduate), and the growth opportunities are promising. But I was recently contacted by a recruiter from an American BB bank, that is looking for analysts for their commodities team in S&T. It seems to be a once in a lifetime opportunity, and just a few months ago I would've killed for a role like this. Although the role is very exciting, I don't think I would be interested in accepting it just now (if I pass the interviews, of course), but I would like to remain on good terms with the bank and perhaps come back to them in a year or so, when I've worked a decent amount of time at my current company. 

Also, I am currently in MO, and usually ppl in my company could progress in about 3-5 years to a trading position, the structure has nothing to do with what you usually have in banks. The role in the bank would be that of an analyst at the trading desk, which seems to sounds better, but I'm wondering what the progression would be. Plus, the bank would be a much more prestigious place to work at, but the salaries are higher in Switzerland, and I'd probably be losing quite a bit in terms of money. 

What do you think would be the best way to proceed? I already had one call with the recruiter, and should have a first interview soon. Should I just proceed and decline in the very end if they accept me? Or better decline early on by giving them a good reason?