Which is 'better' will depend on your situation (eg cash, woodworking ability and so on), and what you are hoping to achieve.
The latest Sigma is more a wide than a full-range driver: it rolls off around 14KHz, so you'll need a tweeter. Therefore, you'll have to factor these into the price on top of the already hefty premium you pay for the Sigma drivers. I like Fostex' own [super]tweeters myself, though most people advise to avoid these and get a ribbon of some form. Horses for courses. The FE208ESigma has a very limited excursion, so it really needs to be horn loaded; the latest Sigma's tend not to work well in other cabinet types (though it's not impossible or unknown to use them).
The FE206E is far cheaper of course, and is a true full-range driver; it's unlikely you'll need a tweeter unless you're particularly bat-eared. It's more versitile in that it can be loaded in other cabinet types as well as a horn, such as a MLTL, TQWT or even BR, though it needs some hefty BSC to tame the rising response. It will also handle more power than its more expensive Sigma brother, but neither requires much grunt to go loud -10 watts will be ample, unless you're using some hefty BSC with the 206.
In terms of tonallity, the Sigma is streets ahead, assuming it's paired with the decent tweeters it needs, unless you don't like any HF at all. It's basically a far more specialised, higher quality driver, but for the price, you'd expect it to be.
Is the Sigma worth the extra? Assuming you can a) afford the drivers + tweeters and b) want a pair of horns, then I'd say go for it without hesitation: you'll need Lowthers to better it, and there are extras like brass-rings you can add to the Fostex at a later date with good results when you fancy an upgrade. I believe Terry Cain has also had some good results with cryogenically freezing these things, so that's something else to think on for future reference.
If you can't afford the Sigmas + required tweeters, or don't want horns, then panic ye not: the FE206E is a great driver for the money, and is very versatile, as is the FE207E for that matter, and by no means a poor choice: It's all the driver a lot of people will ever want. There aren't really many rivals to Fostex in this regard. Supravox perhaps, I've never heard them, so I can't pass comment, but they're not cheap or as easily available I believe (though I could be wrong on this).
Hope some of this helps
Best
Scott
