I think the Galaxy S is going to be way better than the slide. I woudl personally get that it looks like it'll be a great phone.
But I went through the same thing and I already ordered my Incredible. I don't want to wait all summer, especially with the features that are coming out. 1080p video? That will still look pretty bad on a cell phone camera. The phones that are coming out with screens approaching 4.5 inches are just way too big. THey're becoming small tablets, instead of phones. Remember that they're also actual phones, like for making calls. I don't want to be holding a big slab of phone to my face to talk to people. I think the Incredible is perfect sized - big enough for the screen tobe able to show a usable amount of content, and small enough that it's easily pocketable and the like.
Let's see, other features that are coming out by the end of the summer that seem useless. Phones will have 2 GHz processors. Unless Google plans on expanding Android to be (even more of) a small computer platform, these aren't necessary. I would hope that there's no actual need for 2GHz, becuase otherwise my phone won't be able to run whatever that is. But it seems a little OTT for a phone.
Front facing cameras don't matter for me but that's just personal preference.
I don't think you'll have any issues with the Galaxy S, it looks like it'll be TMob's flagship device.