VPS or dedicated server for a web server?
Solution 1:
I have personal experience with Slicehost. They have been acquired by Rackspace a while ago. In my experience, and those of everyone I have heard mention them, Rackspace is professional and reliable. I can also recommend linode.com. Be careful in what you choose, because there are many horrible VPS providers out there.
The biggest reason why I love VPSes is that they are easily adjusted to your needs. So if I get a burst of traffic, e.g. get slashdotted, I can have a machine 10 times as big in a very short period, at any time of day. And, if I downgrade two days later because the traffic is back to normal levels, many VPS providers will only charge the big server for two days. For Rackspace Cloud, I think it's even per hour or per minute.
With physical hardware, upgrades are difficult and lengthy. So, unless your requirements are very extreme, I would always recommend a VPS.
Solution 2:
VPS buys you out-of-band console access, the ability to re-OS, and snapshot backups. Plus your data is on a RAID array not a single spindle. Doesn't mean you'll necessarily get better performance, but less likely to have a "whoops how good are your backups" event.
Rackspacecloud is the marriage of rackspace's facilities and capital with slicehost's platform and community. Its very popular, and imho for good reason.
Solution 3:
Do traffic costs matter?
$0.22/Gb out means that for 150Gb/mo out Rackspace also costs $204
Performance
This VPS performance comparison by Uggedal has been making the rounds
It favors Linode and disses EC2, which fits some experience I've had. I'm happy with Linode.