yes, see my explanation below @spotlight comment.

90 day is a better deal. more bang for your buck.

EX. 10Steem for 30 days gets you ????? 500 SP
10 Steem/30 days for 90 days gets you ??? 560 SP... but that 10 steem is per 30 days... so will cost 10X3= 30 Steem for 90 days.

10-15% better rate.