Definitely go with the pass. Subway fare is $2.65 per trip, so going to and from a place is $5.30. Even if you only go to one place a day for 6 days, that's $31.80. If you use the T to get to more than one place per day, it goes up from there.
Even if you thought you were only going to use it for like 3 days and were thinking "well, 3 days worth of trips is only $16 and the pass is $19 so I'd be saving $3 for each pass", don't. You'll end up going somewhere you didn't expect and having to pay for it. Plus the convenience of it is well worth it; you won't have to worry if you have enough money on your ticket for where you're going every day or do math ahead of time for the things you have planned, you just have a pass and don't have to think about it. The ticket-buying machines in the T stations are, I think, intentionally cumbersome/unintuitive to get you to spend more money on things than you need, so if it's like day 5 of your trip and you need to reload it again, you're almost sure to overpay for it unless you can find the "secret" menu options or whatever.
I live here and have a monthly pass that I technically pay for but my company handles paying for it every month through a payroll deduction and it's simply glorious not having to ever worry about paying for rides or having enough on my pass.
edit: also, if you're staying near the Pru and haven't gotten your passes yet by the time you arrive at the hotel, walk over to Back Bay station and ask the station attendant for CharlieCards, and then use the CharlieCards to put your passes on. It technically doesn't cost less if you're getting a weekly pass, but it's so much more convenient to just wave the card in front of the fare gate sensor instead of inserting a ticket. The ticket mechanisms are really wonky and sometimes take a long time to give you your ticket back.