I've raced on Kashimax single and doubles and actually think the Toshis are a bit better.

The Kashimax's have one of the nicest rivet systems and the best leather finishing. They just look great. The biggest drawbacks with Kashimax doubles is that they are about 3 centimeters or so shorter than either MKS Fit Alpha or Toshis. Most people seem to need that extra length, especially if you like to insert the tip back into the loop on the buckle (standard on the track if not on the road). The leather on the outside is a smooth chromed finish which tends to tear up with use. You can cut it pretty deeply in a power start or in a fall, which isn't nice because these straps are way overpriced (they jumped about 60% in one week a year or so ago, and keep climbing fast). The ends of the straps are nicely tapered, the leather is cut in nice smooth curves (no right angles) and the plastic laminate is pretty much unstretchable. Kashimax straps get frayed a bit from pulling them through slots on pedals. They also don't put up with weather well -- for those of you riding them on the street they will stop looking pretty quickly while Toshi's and MKS's just brush it off and keep going.

The Toshi's have a long double-riveted tab on each buckle which is very nicely done. There's some room between the tabs so the buckle end can shape itself around your foot a bit. There's a longer piece of doubled leather on Toshis than on any other strap, which is a pain if you like your buckle way down on the side of your foot but great if you like the buckle more up on top (which is more common on the track and also keeps the buckle from compressing on your last metatarsal. The leather is sueded (and you can shop around for straps with different amounts of roughness because it varies significantly from strap to strap), extremely strong, absolutely zero stretch. They come in colors, which is nice, although they are coming in fewer and fewer colors these days. The Toshi buckles are also the most rust resistant -- I've had Kashimaxes and MKS's show surface rust and never had it with Toshis (and this is simply from sweat while on the track).

MKS Fit Alpha's are close but not quite as nice in my book. The buckles are riveted with two big side by side rivets. They end up making one continuous strip of metal on the side of your foot, with the two tabs so close that they tend to be stiff and don't shape around the side of your foot as well.

Note that MKS makes two Fit Alpha models. The tan ones are stunning and better than anything out there, but also twice the price of the others MKS straps. I have one pair that I've been riding for three years on the track and they just won't die. They are usually simply designated "tan" and of course have the bigger price tag. They are hard to find now but if you don't mind spending $100 for a pair of singles, you'll have them forever.

Surprisingly, different straps will fit differently on your foot -- these differences in buckle riveting and in the design of the joined leather part of the strap make them settle differently on your foot. It doesn't matter if you're riding them loose but if you want them tight, comfort is a big thing.

Some people with bumps on the outside of their feet or with sensitive feet may do better assembling a double from two singles. This lets you space the buckles a little more or change the relative height of the buckle or do other customizing. Just cut out a piece of leather with two pairs of parallel slots to thread the two straps through.

As for some of the other straps, Somas are very stretchy and don't tighten down properly -- they just aren't worth the money (a single from a better manufacturer is better than a Soma double). Somas are really just for show. Errebi's are fairly stretchy but also tend to delaminate which pretty much kills them. Again, not worth the money. Campy and the old Binda Extra singles are decent but sometimes delaminate and aren't as non-stretch as the Japanese ones. They have become very pricey because people who are restoring vintage road bikes have driven up the prices and they aren't made any longer (not the good ones, anyway). There are various cheaper leather straps (mostly non-laminated) on eBay, but they aren't much more than a way to put straps on your bike for a couple months before they stretch and start to deteriorate. Sad truth is that apart from these three brands -- Kashimax, MKS, and Toshi -- there isn't much out there that's worth the money. It sounds like a crazy price, but there aren't many alternatives between junk and gold.

Notice that I'm not opining on whether to buy expensive ones or not, or on how to ride with straps (or whether to). That's up to you. The OP asked for info, not for personal biases. Hope this helps.