I'll likely do flex for a while, but not LFR.
There's really zero reason to do both. Heroic ToT is 535/541, and nearly everyone will have all their gear fully upgraded before the patch which makes it even an even bigger gap. But even if you have a normal mode item for some slot, you're still looking at it probably being 530 since it's upgraded, and since you're likely to have better things to spend valor on than upgrading LFR items, a 528 from SoO LFR is not going to be useful -- moreso since it won't come out for a few weeks after the tier, so you'll have SoO normal/heroic items to upgrade with the valor you carry over from 5.3.
The only exception would be a trinket or something if it was significantly overpowered for your spec, but even then, you're not going to run all of LFR - just do one wing in lieu of some scenarios to cap valor.
Technically, doing flex will be mostly pointless after the first couple weeks too, moreso the further progressed you are (if you clear normals week 1/2 and start in on heroics, by the time flex is fully open, you'll likely only need an item or two that would be worth downgrading your item level for, if that). After that, flex will be a 100% alt activity, like LFR is now.