The difference between 1.5 stable and beta is much smaller than you seem to think. Unless there's a major gamebreaking bug found in beta update, the release version gets the same update a week or two later.
Besides, it's very likely the Viggen will be released in 1.5 stable with the next update.
As far as buying early access modules, that's really up to you. If you plan to buy it anyway, I see no reason to wait until the full release. If you're on the fence, or can't tolerate bugs and WIP, then yeah maybe you should wait for the full release. Viggen, only a weak after release, is a damn good module and great fun to fly. Sure there are bugs, the bug section has like 10 pages of reported bugs (and we'll sure find 10 pages more :D), but most of them are relative minor bugs, no true showstoppers so far.
Anyway, how could you not want to buy a plane which has the Automatik Fart Kontroll? :D