I recently had three offers on the table at the same time. They mostly coincided, however, one of them was a bit later than the others and I had to stall a bit to make a good decision. Basically, what I did was tell the other companies that I was trying to make the best, most informed decision I could, but that I did not yet have all of the information that I needed to do that. Thats code for "waiting on another company's offer." I've never had a company in such a hurry that they were not willing to wait for a week while I made a decision. Any hiring manager worth a damn knows that it is in the company's best interest to let the candidate make an informed choice, otherwise the candidate might have regrets later and be dissatisfied and more likely to leave unexpectedly. So, while you have probably showed your hand too early to your current employer, you can still push back when they pressure you to make a premature decision. Just ne kind, but firm in telling them that you want to make the best, most informed decision possible and that you would appreciate their patience. Also, before doing that, make sure you have a good idea of how long until the other company makes and offer. If you don't know, call them and ask them, politely, if they have any information. You might say something about having other offers on the table, but that their's is the one you are looking forward to hearing.
Good luck in making your decision. Don't rush it because the last thing you want is to be working somewhere that you don't like. Make sure that you get a tour of the place you'd be working and meet some of the team. Also, ask where you'd sit and what equipment you'd be working on. Find out
as much as possible (details are important) because it sucks to take something like a place to sit or a decent computer for granted only to find that you chose a job with more money, but you're sitting in a shoe box and working on a computer that is past its prime. Not that I'm speaking from experience or anything.