Their are a few things you have to like to like each of the phones, which people have already listed but im going to list them again
you have to like a real keyboard or virtuals. That IMO is the biggest difference in the 2. you know which one is which their.
another thing you have to like is sense (im not sure if its expresso or not, if it has espresso the myTouch slide has expresso. its the same thing as sense just a little more light weight but with all the features as sense.) or vanilla. if you like sense go for the myTouch hd. if you like stock vanilla go for the g2.
Also, is the front facing camera important to you? the mytouch hd has one, the g2 does not.
So basically to wrap it up do you like sense (espresso), physical keyboards, or front facing cameras?
Me myself, i will have to wait till i see the hd's specs. if the mytouch is a g2 with sense and a keyboard delete im going to get the g2. if the mytouch has a 3.7+ inch screen ill get the HD. I like those massive screens!