Hi Thanks you for the advise given : perhaps I have few consideration point here
1) Buying price / New set : F2.8 : S$2130 and F4 : S$1340
2) Buying price / Used set : F2.8 : S$1100-1200 AND F4 : S$1000-1100 ( price from clubsnap market place )
Haven't been up to date on market prices, but the new price of 16-35f4L IS looks good, I would consider buying it new, pay a bit more but have local warranty and less risk
3) Night Photography : only 1 - 2 time annually
4) Photo taking time : sunrise or sunset , does this consider as low light timing?
As most others pointed out, you only need the extra stop of light if you need to do astrophotography stuff like taking photographs of stars and milky way or other genres where minimum shutter speed is limited, like event photography. Otherwise in normal low light / sunrise / sunset situations you normally are on tripod anyway can just use a longer shutter speed to compensate
5) Focus : F2.8 good for low light , F4 excellent sharpness
The Canon 16-35f4L is known for it's sharpness to the corners when stopped down, even at F4 it's still quite sharp. Which is probably more important in landscapes. Another thing is the Image Stabilizer, if you are shooting in low light situations eg in monuments, where tripod is not allowed and the subject is not moving, I found it quite useful
6) Weight : F2.8 : 640gm and F4 : 615gm
I guess not much of a difference?
For the begin 3 years , I'm using APS-C as basic photo taking and this is the first FF DSLR I purchased .
I don't really know how well the FF can perform and renting of lens or camera for test will cause an extra $ , As so I do worry for invest an unpair lens for my photography hobby . Do I have to go for low light F2.8 or consider the price on buying ( F2.8-$2130 / F4-$1340 )and resell ( F2.8-$1200 / F4-$1100 )of both lens ?