Help selecting new foam for lounge chairs.  

Maybe I'm just overthinking this. The woman who is going to do the cushions said she just goes down to JoAnn's whenever she needs foam. I checked their prices, and I think I'd rather go with this stuff online, as it's better quailty for about the same price. I suppose I could feel them if I went to the store.
I have read many threads on here about this kind of thing, but the density issue is what's snagging me. Would a 2" cushion with medium density for the back feel different enough than a 2" cushion with firm density on the seat?
I'm posting some pictures of the chairs, also... ignore the messy basement workspace. Too cold in the garage.

Ugh,  NO foam from Joann's!  It is low quality stuff and will lose its resiliancy pretty quickly, plus there's only one density anymore.  (They call it high density but it's really not.)
Yes, 2" firm is firmer than 2" medium, but not by a lot.  You just won't feel as much of a difference when both are only 2" thick.
These are not high end chairs.  I don't think you need to stick to the original cushion thicknesses unless you really like the look of 2" thick cushions for seat and back.  I would do 2" and 3" or even 3" and 4" if the seat depth can handle it.  It will still look good and it will be more comfortable.
I don't know what to tell you about density.  I don't think the numbers are very definitive.  The feel also depends on the tautness of the webbing, the weight of the people sitting in the chair, whether the spokes of the back can be felt through the foam...a bunch of factors.  You may just have to take a leap of faith.  
I can tell you that 3" thick firm foam on the seat and 2" thick firm foam on the back is usually not enough difference.  The back usually feels too hard.  But again, it's kind of subjective and the best thing is to test out different ones if you can.  Are you near a large city?  Did you check to see if there is a place that sells foam locally?  

Thanks, spanky. I figured you'd be the guy with the best info.
I know they aren't high-end, but I've done a lot of work on them and I want to make sure they turn out as perfectly as I can imagine them. I really do like the 2" seats. I checked all over for prices on foam... the cost of ordering it through foambymail is too cheap not to try... basically like $30 bucks I'm out if I decide I want something softer for the back. 

Perhaps it would make sense to buy a couple of 1 inch thick twin mattress toppers in different densities.  Stock items like that tend to be much cheaper than custom cut foam pieces.  Then you can test stack them in different orders and combinations and perhaps find an arrangement that suits you.  Won't be an issues that it is two pieces of foam inside the cushion because it will be wrapped in dacron or cotton or somesuch.

Um, mgee? In addition to freely offering some great advice here based on sound judgement and long experience, spanky is about as good-natured a regular guy as you'll find... for a lady.
And she's far more polite than I am. I recommend reconsidering her suggestion on the thicker cushions.