Hmm... I'll have to try this test later today. We live in a suburb of DC (where there should be HSPA+ coverage). I still have an iPhone 4 and my wife has an iPhone 4S, so we could test at the same location and compare the results. I'll report back with an edit to this post...
EDIT: Testing done. Did 3 rounds, each simultaneously with the iPhones about a foot from each other. Both phones showed 3 (out of 5) bars of signal.
Format of results is Ping (ms), Download (Mbps), Upload (Mbps)
iPhone 4S:
1: 88, 1.46, 1.24
2: 88, 1.17, 1.04
3: 78, 2.96, 1.48
AVG: 84.6, 1.38, 1.25
iPhone 4:
1: 152, 2.01, .78
2: 156, .77, .54
3: 166, 1.29, .29
AVG: 158, 1.36, .54
So, clearly the iPhone 4S is the winner in my test, but not nearly by the margin others are showing. The biggest win though is in ping, which the 4S is getting nearly twice as good results. In my experience, ping has been a major differentiator between AT&T's network and other networks such as Verizon and Sprint, with AT&T posting traditionally good throughput but very high latency and the others not offering as much throughput but having very good latency (it used to be common for me to get 300+ ms pings on AT&T 3G). It appears that AT&T has now vastly narrowed that gap and is putting up much more respectable numbers.