The reroll is probably better. The benefits of a reroll are a bit better than what the average roll results would imply, assuming that you're not running into a ton of saves on the far end of the spectrum.
If you would fail a save that you need to roll X or better on to succeed, the odds that a +1 would make it a success instead of a failure is 1/(X-1). The odds of a reroll making a success is (21-X)/20. Assuming you need the same number on every save during the day, a +1 bonus is worth a single daily reroll (in terms of average number of saves improved to successes) after (21-X)(X-1)/20 = (-X^2 +22X - 21)/20 failed saves.
The +1 doesn't help if you'd normally need a natural 1 or 2 on to succeed, because then you succeed on a 2 and fail on a 1 either way. Likewise, a +1 bonus is also no help if you're still only succeeding on a natural 20 after applying it.
min saves
roll +1 bonus reroll /day
1 0.0% 95% inf
2 0.0% 95% inf
3 50.0% 90% 1.8
4 33.3% 85% 2.55
5 25.0% 80% 3.2
6 20.0% 75% 3.75
7 16.7% 70% 4.2
8 14.3% 65% 4.55
9 12.5% 60% 4.8
10 11.1% 55% 4.95
11 10.0% 50% 5.0
12 9.1% 45% 4.95
13 8.3% 40% 4.8
14 7.7% 35% 4.55
15 7.1% 30% 4.2
16 6.7% 25% 3.75
17 6.3% 20% 3.2
18 5.9% 15% 2.55
19 5.6% 10% 1.8
20 5.3% 5% 0.95
21+ 0.0% 5% inf