This has been covered recently, but use whatever kind of miso you want. Generally speaking, the darker it is, the stronger the flavor. I use both aka and shiro. And yes, miso will keep for a LONG time, so go ahead and buy both.
It's well-discussed here:
There are almost no mistakes with miso: It lasts forever; all sorts of types are yummy. If you buy a higher grade non-pasteurized miso, then keep your stock temperature below 160 F to avoid pasteurization and to let the beneficial things make their way to your belly.
re: Jimmy Buffet
Jimmy, can we assume from your moniker that you are located in Key West? I will gladly trade you a pail of miso on Tuesday for a bag of Key Limes today.
There are many online distributors of good misos, if your local health food store does not carry them. My local Asian store carries only mainstream pasteurized miso, though if you find only that it will give you a good introduction to miso's flavors.
Best artisan brands (unpasteurized) that I know are Mitoku, South River, and Miso Master (distributed thru Great Eastern Sun).
If you're in Denver, get some of Taki's miso paste. Taki's is at 341 E Colfax Ave, though I think the miso is now also sold in places like Whole Foods as well as in the restaurant. I can't find a website for Taki's but have to wonder if he's started selling his miso paste beyond Denver.
I made some recently with South River Miso's 3-year old hearty brown rice miso and was amazed at the depth of flavor.
Mitoku miso: I've ordered thru these folks:
This South river site is wonderful for clicking around and getting a feel for the production and varieties of traditionally made miso.
As to which one is best for soup: if you get "shiro", which is the mildest and youngest, you will not go wrong. One step up from there is aka miso. If you are ordering online, and want the best bang for you shipping-charges buck, get four: shiro, aka, mugi, and hatcho, listed in ascending order of strength.