I've already talked to my current employer about the situation, and they're working on a raise/increased responsibilities offer which may include bringing some of the bookkeeping in-house. I should know that by Wednesday/Thursday, since I have to give an answer re: the temp job offer by Thursday night. At a minimum, I know they're going to pay for me to get Quickbooks certified.
It sounds to me like you have the potential to make the best of both worlds. If your current employer wants to make an effort to keep you, than it sounds like you have some room to negotiate for what you want. If I were you, I'd really prepare for your meeting with your current employer. I'd make a list of the positive things the new job offers you (higher salary, more responsibility, flexible schedule, etc) to present it to your current employer (keep the negative things to yourself, they don't need to know those ;-) ). I personally think one of the most important things to think about is the potential to improve your resume and career path. It sounds like your new job currently 'wins' there, but that's something that your current job could meet just by giving you some training, a new title, and responsibility.
Then, for your use, decide what you would like your 'perfect job' to look like if you combined the best of both jobs and prioritize what's most important to you. I'd also put together the numbers so you know what is the value of each job perk. How much tax savings does the 401(k) provide, commuting cost differences, etc. The hardest ones to put a number to will be insurance costs, but you can still put a good guess on it.
In your meeting, you can present the things that are important to you with the new job. While your current employer will probably only be willing to raise your salary a set amount, you may be able to get a bunch of intangibles that don't 'cost' them nearly as much. Things such as formal training, a title change, and flexible working schedule don't cost the company a lot but may be worth a lot to you.
I doubt it'll happen all in one meeting, but you should be prepared in case it does. In the end, the employer will offer some package meeting some of your demands. You can compare it to your 'priority' and 'value' list to see if they provide enough incentives to make your current job more worthwhile. If the salaries are really as close as you think they are, it seems like you're in a position to get a significant pay bump and may be able to get some other incentives as well.