I have both. I used the Moment for 6 months and I am so into Android technology, I wanted my teenager to have one. I ordered the Hero because I didn't think my teenager would care if it had a keyboard and it was cool looking and stylish. My son decided he wanted the physical keyboard, so I gave him my Moment and I adopted the stylish Hero.
The Hero is smaller and easier to fit into a pocket or purse. I prefer the calendaring options on the HERO and the battery lasts a few hours longer during the day. I'm in a weak reception area at work, so the HERO picks up a signal better.
After a month, I"m missing my Moment. The keypad on the Moment (no matter how bad they say it is) makes typing in addresses, responding to messages so much faster. I can do it with a HERO, but since there's no physical keyboard it takes longer. I've been lost, flipped open my Moment and typed and had directions, quickly.
Call quality is life like on the Moment. Call quality is good on the HERO, but it's better on the Moment.
The Moment is much faster, but I'm OK with speed of HERO. I can download 5 or more items, simultaneously, with the Moment and it zips thorugh with ease. With HERO I try not to do more than 2 or 3. This isn't too big of a deal for me, because once something is downloaded I'm done.
Sense UI - I mean it's nice, but... you have to decide if you really have to have this. It's not a must have for me.
Android 2.1 breathes new life into Moment. Gives it a new personality. It is as if the 2.1 system is maximizing use of faster processor on Moment.
My daughter is due for her Android in a week. I may give her my HERO (she likes the style) and go back to the Moment or I may stick with the smaller footprint of the HERO.... I don't know.
Moment is heavier. Screen is nicer for viewing videos and such. THe Moment will make phatom phone calls about once in 5 days. This is annoying, but not a deal breaker for me.
It depends on your preference and how you expect to use your phone.
I would lean towards the Moment for the keyboard, AMOLED display and faster processor.