I went with Tampa.
Yes, the regular season record seems to suggest that the Bruins fare much better against the Leafs (4-0-0 vs TOR, as opposed to 1-1-2 against TBL). But the regular season is weird, both in terms of teams meeting each other at weird times (uneven rest, injuries, slumps, etc.) and of course the different OT rules. The Bruins had a pair of 4-1 regulation wins over Toronto but just 3 days apart, and the other two wins against the Leafs were an OT and a SO win. Meanwhile the only regulation loss to Tampa was the end of a back-to-back on the road after being the Panthers the night before. Remaining outcomes versus the Lightning are one OT loss, one SO loss, and a 7-3 win back in January.
On paper Toronto is a much better team, and I think a seven game series would bear this out. Leafs are 2nd in scoring (3.65 GF/GP) while the Bolts are 5th (3.48 GF/GP), and the Leafs are 18th in preventing goals (3.13 GA/GP) while the Lightning are 22nd (3.25 GA/GP). As for goaltenders, Vasilevskiy has been below average (-0.049 GSAE/60, 48th overall of goalies with 10+ games played) and his backups have been catastrophic. Meanwhile Woll has been good (0.264 GSAE, 16th overall) and Martin Jones even better (0.448 GSAE, 8th overall), so I really don't understand their love of Ilya Samsonov and his -0.211 GSAE/60 (ranked 60th).
The only area that Tampa has a clear advantage is on special teams (1st on PP, 5th on PK) over Toronto (7th PP, 24th PK). But playoffs tend to suppress the importance of special teams...