Going through the same internal debate
If we only look at the price, then f/2.8 GM does seem a lot more attractive than the f/4 PZ one. If we take into consideration their respective weights as as snerkler stated above though, the choice becomes less obvious. The GM one is almost twice as heavy at 680g as the PZ one which weights 353g.
For landscape photography, more often than not, f/4 aperture should be good enough but I like the flexibility of having a bigger aperture for versatility as well.
The 16-35mm GM mark II is rumoured to be coming out in March/April according to the Sony Alpha Rumor website which may have a knock-on effects on the mark I price if you're not too much in a rush