Can't really go wrong with either one.

M10 = Hand Ejector, Military and Police, being made since 1899 non-stop, an iconic firearm, the quintessential revolver.

M15 = Essentially a M10 with adj. sights. Called the Combat Masterpiece.

Just listen to that... Masterpiece.

You obviously need both. And you have reminded me how much I need an M15 myself.

PS: Don't forget that you also need an M19 - Combat Magnum. (Basically an M15 in .357 Magnum).