Y'know, I'm not actually sure which order it was applied in the MW roller. Over there when I saw this roll, we used 4d6v1r1z repeated six times. I suppose if operations were performed in the order of the string, that'd drop lowest first, then reroll 1s, but no way to know without getting into MW roller code heh.
And I'm not sure if the order matters either. It's been a long time since my stats classes haha. But feeling-wise, I'd agree it feels like it'd give slightly higher results if you rerolled 1s first, and dropped lowest second. Because if you originally rolled a 1, that 1 is obviously the lowest, but when it's rerolled the new value may end up higher than one of the other dice that would've been kept, giving you a higher result than you would've gotten if you dropped the 1 first. Also if you drop 1s first, you're only rerolling at all if your original roll got multiple 1s... hm. Experiment time!
So example, original roll was 2, 3, 4, 1 on a d6. Reroll 1s first maybe gives you a 6 so now it's 2, 3, 4, 6. Drop lowest loses the 2 so your final total is 13.
Same original roll 2, 3, 4, 1 on a d6, drop lowest first loses the 1 so now it's 2, 3, 4. Nothing to reroll now, so final total is 9. Diff would've been smaller if reroll was < 6 but still.
So let's try something like, original roll 6, 6, 1, 1. Reroll first gives let's say 6, 6, 2, 3. Then drop lowest is 6, 6, 3 = 15.
Then original 6, 6, 1, 1. Drop lowest 1 gives 6, 6, 1. Reroll the one, gets 6, 6, 2 (same 1st reroll value from above) = 14. Less difference but still lower.
I can't think of a scenario where dropping the lowest first ever could give you a higher value compared to rerolling 1s first, only same or lower. Interesting. So yeah, unless someone can come up with that scenario, I'd go with my gut and say rerolling 1s first gives a slightly higher average than dropping 1s first. xD
*edit* Ahh, wait. In that second scenario above with 6, 6, 1, 1. If the reroll of 6, 6, 1 gives something higher than 3 (let's say 6, 6, 4) then your final total is 16...
Although, assuming the diceroller would give the same value for the 'first' reroll that would also give us 6, 6, 4, 3 --> 6, 6, 4 = 16 on the reroll first one as well...
Hrm. Complicated. But maybe it does average out so close to the same it doesn't matter. xD