At the beginning of this month, I, too, was trying to decide between Site5 and DreamHost. I ended up going with Site5, and here's why.
I'm on Site5's Horsepower plan, which ended up being $6.95/mo. for me since I paid for an entire year in advance. Had I chosen the same contract length with DreamHost, it would've cost $9.95/mo. While it's true that you get more bandwidth with DreamHost, my site is at the point right now where I'm not even going past 10 gigs a month. Site5 gives you 3 GB of space and 50 GB of bandwidth, but DreamHost gives you 2.4 GB of space and 120 GB of bandwith.
If you're thinking about expanding to more domains in the future, Site5 gives you 5 fully hosted domains, whereas DreamHost only allows for 3. DreamHost limits you to 15 subdomains, but you can have as many as you want with Site5 (personally, I'm not sure I'd ever need more than 15 anyway). You'll get your own IP address with Site5, but DreamHost will charge you $5/mo. extra for that.
Recently, I've heard some things about sites hosted with DreamHost being slow, especially when dealing with MySQL. I can't personally confirm this since I don't have an account there, but I can tell you that I haven't yet experienced any such issues with Site5.
Both services offer shell access via SSH, which is important to me, since I'm more comfortable using the command line than I am using a GUI. If what I just said doesn't make any sense to you, just keep reading.
Uptime has been decent so far at Site5, but bear in mind that I've only been with them for about a month. There was an unplanned outage a couple days ago where several services went down, but it was a result of a high server load, and it was fixed within an hour or so. All my other support tickets have been answered within two hours--even the ones that were initiated at odd hours of the night.
Dreamhost has a promotion where it's only $9.24 for web hosting & a domain name for 1 year. The promo code is 777. I can't comment about their service as I've not used it. A co-worker of mine has and says they're pretty good, but can get a bit slow sometimes. I guess it's understandable when you're giving away 2400 mb of space & 110 gigs of bandwidth for a year for free.
Site5 is AWESOME. They do give you credit if you are switching, and the service is excellent. They also just started a new program called superhosting with their customized control panel, which I am VERY excited about. (go to site5 for more info on that.)
Also, if you don't believe their 15 minute average on support tickets, I've sent them more than six tickets and they've answered all but one in under ten minutes (the sixth one took a day, but it was when they had just revamped all their services and support was flooded.)