Go with a 15+mini as both are somewhat redundant in their throw and cutting ability. I've got both but ended up purchasing a FLEX for heavier correction. I use the FLEX to compound, Duetto to polish/finish, and Mini for tighter areas. Also have a PC from ages ago to do beater work.
I have the 3401 and the pc as well. The reason I'd like the Duetto, and the Mini, is that they're similar throw/design. I feel that they would be a compliment to the Flex. I also would like to pick up a Flex rotary someday.