Right now, I have to say Solinsky's 10K, but I'm not sure if that is simply because I am riding the emotional tide of the more publicized record accomplished on American soil. However, the fact that he is now the only non-African sub-27 runner remains a compelling argument...
I think Solinsky because his race was like El Guerrouj setting the WR for the mile where the winner would have the record. Rupp was Noah Ngeny inn this race. techrunner
This is a thread that I wanted to start, so I'll jump in. It's hard to say, I think. Solinsky's 10K has a higher rank on the all-time lists than Ritz's 5K, but the 10K is also run much less frequently. There are a dozen or more high-quality 5Ks on the circuit each year, but maybe 2-3 10Ks. The differentials to current World Records are exactly comparable (20 sec and 42 sec). I think all of these are a wash.
I give the edge to Solinsky because of the nature of his race. He won the race, and his closing speed was competitive with anyone in the world (last 800 in 1:56).
LetsRun actually has an equivalent performance chart here:
http://www.letsrun.com/2007/jkconversion.pdf
According to this, Solinsky's time is equal to 12:56.24 for 5k. Ritzenhein's record is 12:56.27.
Solinsky won his race outright but it wasn't a Golden League event that had the greatest active runner (possibly the greatest ever) on the planet in it. He also sat close to the leaders and took over pretty close to the end, while Ritz had to work his way up through the field running his own pace and had a fantastic last 4 laps to pick off some real studs and finish 3rd at Weltklasse. Solinsky was running his debut 10k, but Ritz took 20 seconds off his previous 5k best. Pretty equal in terms of breakthroughs. Competitive edge probably goes to Ritz for his placing in the Golden League race, but historical significance probably to Solinsky as first non-African under 27:00. It's a tossup.