I have no preference for anything special since I use both cortland products and some other off brand name. They have not failed me yet. Good luck with chosing your backing. I honestly don't think there is much of difference from what I have seen in the products other than the name (IMHO.)
I see no reason to use anything other than 30lb braided dacron or micron -unless you can't get the capacity you need, in which case you might want to use one of the gel spuns. Having said that, I'd add that your reel is probably a bit undersized if you do actually need to use the gel spuns to get capacity.
I use 30lb Mason or Cortland braided greenspot trolling line. It always seems to be quite a bit less expensive than anything that is labeled as "fly line backing" and I like the look of the green checks a little better than standard white.
as for diameter gel spun rocks. the argument is that if not put on REALLY tight it might lay over it self and cause big problems when the big one hits. i use use it anyways and have never had a problem. 350yrds looks like nothing on a reel. for saltwater it really helps and many say 300 yards minimum. if you are light lineing your set up you will get into it . (skip jack on an 7 wt.) what kind of fish are you geting into? i recommend the yellow 30lb. tuffline xp. (300 yards for around $20)
I'll forwarn you about using those gelspun lines. I'm going more off Powerpro, but is pretty much the same thing. If you don't carefully watch that line as you're putting it back on, you could have MAJOR problems with your reel AND rod. Nothing worse then losing the fish of a lifetime while breaking the rod too.
I've been using powerpro for a couple years now. Mostly on my baitcasters and spinning rods. Have used it a little in flyfishing, but not much. The problem with this stuff is the diameter. First, you have to do a TON of cranking to wind that stuff back on. Unless you're deep lining the salt, you won't need 200+ yards of backing (just think having a fish being TWO football fields away, usually won't happen and if it does say goodbye anyways lol). But the stuff is prone to wrap around it self. If you're fighting the fish and it's putting pressure on the backing, it'll slowly dig it's way into the reel/line. Bascially making a ratsnest (which is usually reserved for baitcasters). You'll have a good run of the fish and the line will stop. Problem is, it's strong stuff and just won't "break" on you. If fish is fighting hard enough, *snap* goes the rod. I had a similar experience last year fishing for salmon from a jet sled using powerpro as backing (helps having a friend who's a prostaffer for them lol). Had similar problem, luckily it was only on a 30# king, would've hate to have a big brute on the end. Luckily, I was using a glass rod, so took the abuse a bit more while I tried to free the spool. Thank god for years of working a baitcaster, i was able to pick the spool out quick while stripping in line. But wasn't pretty, and those superlines will cut into your hand while stripping in a big fish (if you are were in the position I just described where you can't really reel).
I am back just using standard dacron backing in 30# test. No need for all the extra room. Plus, when I'm cranking line back on, I don't want to have to think that much about line placement. Which is critical for this stuff.
I use 30lb. backing, but all 30lb. backing is not the same. I prefer Micron since it seems to have the smallest diameter available and can be blind spliced. I tried to blind splice SA dacron backing, but couldn't get the spliced line to hold. I have purchased the 30lb. backing that is offered almost daily on eBay, but the diameter appears to be much, much wider than either Micron or SA. MKy guess is that I won't use it for steelhead or salmon fishing. My $0.02.
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