Food for thought...
The G19 wins on weight; Glock's website says 21oz unloaded and 30oz loaded, although the latter weight will depend on what load you're using. The M6906 weighs 26oz loaded and probably ~33oz loaded; I don't have one and can't find a loaded weight spec. Remember that one of the major advantages of modern tactical plastic is lighter weight than all-metal pistols of roughly equal capacity, even those with alloy frames like the M6906.
Despite the lower weight, the G19 will probably have lower felt recoil- although this is somewhat subjective- and will definitely have less muzzle flip. S&W TDA pistols have high bore axes and categorically don't do well on muzzle flip.
The G19 wins on mag capacity AND availability- 15rd vs. 12rd, and Glock mags are available everywhere for cheap (one day I expect to see them in blister packs next to a gas station cash register).
Some inexpensive 69-series mags are floating around, but they're still nowhere near as plentiful as Glock mags. The M6906 will accept 15rd 59-series mags, but they will protrude beneath the grip frame and negate the pistol's size advantage.
If you're looking at particular pistols, early-production M6906s had a vertical-blade rear sight has less sight radius and fewer replacement options than the later Novak sights, and had a squared-off trigger guard that causes issues with holster fitment; as with other 3rd-gen Smith pistols, round trigger guard pistols will fit in square trigger guard holsters but often not the reverse, so it's generally easier to find holsters for the rounded guard. Myself and other TDA owners have found that this factor isn't always reflected in holster maker's catalogs.
One issue with holster fitment for CCW with a M6906 may be the decocker/safety levers. I sometimes carry a M3904 and have had issues with the lever digging into my side if the holster doesn't cant the pistol slightly away from my body or incorporate a guard flap to protect the lever.
All that being said, I would get the M6906 because S&W 3rd-gen pistols just make me happy.