Like I said before, it'll be tit for tat, back and forth between the two for a good chunk of the year. I can see it going like this:
January: XBO (simply b/c of having the supply, but I'm thinking it'll do below 600,000 even with that supply).
February: PS4 (even w/ Japan launch, I just don't see XBO demand helping it sustain near 600,000. Probably closer to 450,000. PS4 will have enough supply to beat that in the States).
March: XBO (TitanFall should help give them a boost).
April: PS4 (TitanFall will do quick big sales but drop like a rock after a couple of weeks, maybe have a slow burner effect. Sony's big games tend to have some legs so if DriveClub and Second Son are out by now they will help push systems).
May: PS4 (Don't think either system has big releases that month; PS4 launch demand will have totally subsided by this point).
June: PS4 (I think E3 will have a few megabombs, and something's telling me there'll be something very worth the wait for PS4 revealed there. Also PS Now will be going full scale, that should help).
July: XBO (MS might try a split SKU model around this time; casuals don't tend to buy console during the "slow" summer months and I think by this point PS4 will be more appealing to core/hardcore gamers still holding out for stuff aside from TitanFall)
August: PS4 (Especially if The Order comes out around this time)
September: PS4 (If Quantum Break comes out during this month, I can see XBO taking September)
October: PS4
November: PS4 (I think Sony will probably do a price cut on PS4 around this time to coincide with the holidays and take back the price advantage instead of being at price parity w/ MS. MS won't be inclined to do a price drop b/c they would have already done one back in July w/ the split SKU w/o Kinect; XBO should still do pretty close thanks to Halo 5).
December: XBO (Just an eerie redux of 2013; PS4 sales in November would likely cut down on December supply due to the price drop especially if it's done in the first half of the month, leaving less supply of them vs. XBO. That should benefit the XBO).
I still see the PS4 winning NA for 2014 when all's said and done, but it's not going to be the landslide some think it could (or should) be.
I could be 100% wrong of course, but sorta doubt that