Oring Monkey: More expensive, harder to use without chart
Marker Kits: More expensive, took forever to get it, harder to use.
Cheap, Quality, Ease of use
I've bought oring kits from several online stores before. I've never had a bad experience ordering from anyone before, but I've never had a particularly "over-the-top" experience either. I bought this kit from them when I realized they were cheaper than anyone else and I figured I'd give a new company a shot. I ordered Tuesday morning and had them Wednesday afternoon (I live in NJ, they are located in NJ). This means they shipped the same day that I ordered.
When I got the kit I noticed they included a nice o-ring sizing chart, as well as a list stating where each o-ring goes in the marker. Very easy to follow, and the chart eliminates the need to measure o-rings. My gun was in need of a full rebuild, and it took only a few minutes to be back up and running.
If you need orings, buy from here. From what I've figured out about them, they are a fairly new company, but I can tell from experience they hang with the well known guys with oringmonkey and markerkits.