To answer your first question, I think it depends greatly on which resource you have the most of. The early hordes can probably be handled (if you are actively killing them) with just upgraded wood, but only if you can get enough wood to upgrade your wood frames to at least level 2. Later hordes will likely tear through this easily since the hordes would be much larger.
If you start mining early (ie get a decent level pickaxe as loot, perhaps) and get tons of stone, I'd recommend going the cobblestone (or cement/concrete if you find the book) route. I usually don't use cobblestone, so I'm not sure if they can be later upgraded to concrete. I did test last night that regular a concrete block can be upgraded to reinforced concrete with 10 concrete mix (no rebar frames required).
To reiterate, I really think it just makes a difference which material you have more up between wood and stone. If you're in the desert, for example, I'd choose the stone/cobblestone option. If you're in a forest with tons of trees, I'd go the wooden (upgraded wood frames) route. Just remember to upgrade either of them eventually for later hordes.
I also make a point to plant the 2 maple seeds whenever I cut down a tree (close to my base) so I don't find myself without a wood source when I really need it.