Important Career Decision - HELP?

I have been temping with a company for 6 months. I had hoped to be hired on but it didn't look promising because of no open positions in my field. I recently applied to another company - a VERY good one and got an interview. I am waiting to hear back from them. However, on Friday the company I am temping for - also a good one but it has no room for advancement in my field beyond what I would get hired into - offered me a job. I am in negotiations with them right now and they will probably want an answer from me on Monday. But I haven't heard back from this other company yet - and I think I have a shot but really who knows. Would it be wrong for me to accept the position at my current company and then quit if the other company offers me the job I REALLY want? I like both places but the other company is better for my field, though I feel a loyalty to where I am now and don't want to make them mad but I really need to look out for myself. Please help!