Interesting question, if the plan is to carry them around and sell them on your return.
Laser-mining of Platinum is certainly normally the most efficient in terms of credits/hour, though as with a lot of laser mining the really high yields are relying on a HazRES to boost it and allow mapping. Still, even in deep space you should get something decent.
But Platinum certainly isn't the most valuable commodity per-tonne - Monazite and other core-mined gems can sell for 2-3 times as much per tonne, so if you're still planning a long trip, or don't have a huge amount of free hold space, doing it that way will get you the biggest sales at the end for the same size of cargo hold.
There's also the option to mine Tritium (generally with subsurface missiles, though laser mining is possible but slow) - it doesn't sell for anywhere near as much as the others, but you might need it anyway depending on your fuel budget. Combining subsurface Tritium mining with core-gem hunting in an icy ring might work out quite well, and it does have the advantage that you might be able to sell it while still in deep space to other players for much more than you'd get at the bubble's NPC markets.
Whether it's worth your time probably depends how much you want to go mining to provide variety in the exploration. (You'd always be more "efficient" - since the mined goods wouldn't be weighing you down - heading back to the bubble without, and then mining when you got back. But if you want to go mining now, go mining now.)