Really it depends. Retirement stuff gets tricky because you're still taxed, but it's when you retire. Then again, you dodge capital gains tax by putting money in there (if you would have to pay capital gains tax).
Personally, I'd want the loan out of the way. The only situation I can think of where I'd rather invest is if next year I'll be maxing out my 403b, so I wouldn't be able to put the money I now have available because the loan is paid off into a tax shelter. Does that make sense? If I was planning on contributing $18k to my 403b next year regardless of the loan status, I'd try and get more into it this year even if it means slowing down on the loan. If I was only planning on contributing $5k next year, I'd just pay off the loan, THEN start putting extra in the 403b. Basically if you're missing out on 403b capacity by waiting until the loan is paid off to contribute, don't wait. It doesn't matter much whether you put the extra $7k or so in the 403b this year or next year.