I've used the SR03 guts. They're very similar (perhaps identical) to the GK Edge E4 reed and tone board. I couldn't detect any difference by measuring them. They're easy to run, not requiring a lot of air. Personally, I have grown to prefer the Saunders red or blue guts, with the tone board modified to allow the reed angle changed to allow the reed to sit closer to the tone channel, since I don't have a lot of air volume. I can get the modified Saunders guts to break in quicker and they have a bit different sound to them.
But, the SR03 guts, as mentioned on the website, are very popular and work well in most calls without modification. You can also use the GK Edge E3 or E4 reeds with them, if desired. The narrow end of the E3 reed will allow you to get the tone board broken in faster. Or, if you're only going to buy one set ... you could get them, E3 or E4 from GK Calls. The per gut set cost is higher, but shipping costs are lower. If you buy several sets, WEBFoot's unit price will save you money.
If you just want to try several different guts sets, you might look at WEBFoots sets which are reported to be popular with his customers. There's usually a good reason that some sets are more popular than others.