FWIW:
Don't compare signal bars. They're relative to the manufacturer.
On your iPhone, if you dial *3001#12345#*, it will put it into Field Test mode. You'll notice that your bars of signal has been replaced by the actual signal strength RF reading (where the lower the number, the stronger the signal).
If you're comparing phones, google how to put the other phone into its Field Test mode and then compare the two actual RF readings.
If kdarling were here, he'd be able to correct all of the lingo above with correct terms, but you should get the basic idea.
My experience:
Original iPhone = 1 bar of GSM/EDGE reception in my house. Indoors, could only make calls from one spot in living room. Calls were often garbled for extended periods of time and dropped almost every time. Safari would not work, even though the E showed up. Sending text messages sometimes failed.
iPhone 3G = 1 bar of UMTS/HSPA reception in my house. Indoors, can make calls from anywhere. Calls rarely garble, and if so, only for a second. Calls don't drop. Safari works fine and seems faster than a good EDGE signal. Text messages never fail.
This matches what I've read over at Howardforums for the last year+ ... 3G (UMTS) voice sounds better on 1 bar than GSM does.