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I'm going with 3m here. The 3467m is redundant, so discarding from it leaves the widest path to iishanten, and the iishanten shape will be great too since the 33488 will remain intact. The furiten at 25p is annoying, but chances are you won't end up tenpai on it and even if you do, you can go with the 38p shanpon wait instead. Of the 3467m, 34m is better because iipeiko and 4m is safe against south, so let's dispose of 3m now.

http://tenhou.net/2/?q=3466778m43388p67s

The tenhou claculator say the same as you do (except it doesn´t consider yaku apart from Chiitoitsu and Kokushi). The two tiles with more tiles to choose from when discarded are and . And I too would had choosen , maybe , which is not so bad.

As you say, The 25p furiten is annoying. but since it doesn't seem to be of major concern to you, why not just discard the . This keeps the in play (three remaining) and only removes the two remaining as good tiles.
I was bothered by the furiten so I tossed the right then and there to eliminate it. I also didn't feel comfortable tossing any of the cracks. Seemed like a lot of possibilities there though I might still end up with the 38p shanpon wait.....or not
What do you think?

You lose 8p as well if you drop 3p. Seven pairs too - drawing 67s can put the hand into iishanten for that without any further drawback to tanyao pinfu. Furthermore, by discarding 3m, you can simply discard 4m when you get to iishanten, losing nothing. But if you discard 3p or 4p, you will have to reduce your waits again in iishanten. The 33488p shape is very useful.