I would go with the 3215 due to the larger size and Independent
PTO. It weighs almost 700# more than the 2415HST, plus you'll get a 60" bucket instead of a 48".
Here in MS we charge sales tax on everything....normal rate is 7% but farm tax is 1% on tractors and 3% on implements. In the state of MS I don't believe a finishing mower is considered a farm implement though so it wouldn't qualify for the 3%. It's hard to say, all this stuff varies quite a bit from state to state.