I had a few of the Tapco's, but traded them for something useful.
My problem with the DuckBill mags is the following:
1: The Bolt has to be back in order to insert the mag (this can be a hassle in practice).
2: Un-modified, they either don't work at all or work poorly when feeding from stripper clips.
3: They make the rifle bulkier without actually adding capacity (gotta love/hate that pop-rivet law).
Having said all that, I might end up stockpiling a few of the metal 5/30 rounders. If you poke around YouTube, it isn't hard to find videos showing why the stripper clip issue exists, and how to fix it - it involves a little bit of grinding on the real of the lips.
I would prefer the metal 5/30 over the polymer 5/20 or 5/30 for the aesthetics, also, the only functional reason to get the mags is "In case of SHTF/ZomPoc, pop the rivet and rock and roll"... Without getting too specific, it's an order of magnitude easier to pop the aluminum rivet from the metal mags than it is to dis-assemble the TapCo mags and chop out the the internal follower block. Again, for procedure, YouTube is your friend.
As always, knowing how to modify a mag is legal. They haven't outlawed smart (yet), but actually doing it is a quick way to get 3-10 years free room and board.