The Mercury dimes are older and much cooler but for stacking reasons I would stick with the Rosies ever since there was a Video put out by Youtube stacker that since Rosie dimes are newer and less wear they were simply a logical choice. He proved this by weighting a Roll of Rosies versus a roll of Mercs and the difference can be significant.
@kerrislravenhill - Thanks for that info! That sounds crazy but true! I would love to get some Roosevelt silver dimes... next time I'm at the coin shop I'll be looking for them. Cheers!