They're the same headphone more or less.
One just has the 4-conductor cable and one doesn't. That's all. All the prestige series is mostly the same.
The SR-80 has better and tighter (translates to faster) bass due to the cable (apparently), but it was mostly with the bowl pads they used to issue as standard back in the day. It's minute, if you've gotta go hungry to get them, just get the SR-60's, I mean hell they come with the same pads these days.
As for "i" vs "non-i" models, I own an SR-80i and an SR-60, both in stock form (now modded with the same cable) with bowl pads pretty much are indiscernable to me, the new cans look better but I don't hear a difference and I've been owning and listening to Grados as long as I've been on this forum.
I'm in class but here is my facebook picture with the 60's, sorry you have to look at me, but there they are lol.
My ORIGINAL pair of SR-60's that I still own today:
Buy whatever you can afford, if the SR-60's don't impress you, I highly doubt the SR-325's will either, all the more expensive Grados are just more of the same with tweaks made to allow the drivers to function properly, and of course American labor tends to be overcharged for but not overpaid... but they'll still be able to compete with headphones in their price range. Their ability to sound better than headphones costing 2-3x as much is grossly exaggerated, but it is true that some people prefer them to more expensive cans.
By the way the comfort issue these days is mostly moot, the build-up of anti Grado comfort posts in the last 10 years or so was due to the SR-80 and above shipping with bowl pads. These days you get the soft foam pads which can be quite comfy compared to sweaty pleather. For example ergonomically my AKG K 240's were awesome, those **** pads made my ears hot, I'm not fat, I'm not overly sweaty, they just irked me but I mean I'm wearing headphones, just take 'em off every 30 minutes lol.
That being said the AD700's have LESS bass than SR-80. You can hear it just as well, but you'll "feel" it more with the Grado. Both are about 1/4th the exaggerated poo-bass you get from Skullcandy Skullcrushers, I'm not kidding, but they are very musical and they are one of my favorites for sure. AD700 is one of my top recommendations as well, but if you're curious about Grado headphones, might as well just kill the cat and order 'em buddy.
1 more tip, get the Alessandro MS-1's, all the features of the SR-125 for less money plus you still get a 3.5mm plug as stock equipment with the option to use the 1/4" adapter. The highs aren't as violently bright (allegedly) but it's still 95 percent of the Grado sound. I honestly just think it's the SR-125 features doing it since I also have an SR-125, but I loaned it out and I don't really want to ask for it back since my friend loves them so much