what are the advantages/disadvantages of cloud server versus a VPS?

I was looking around for inexpensive linux/PHP hosting for a project that is still in development. I got some recommendations for slicehost. But then a freind mentioned rackspace cloud servers, and this looked even better. First it's cheaper (assuming I won't have a lot of bandwidth to begin with). Also it seems quite flexible and simple, i.e. being able to backup and clone an entire server, and just turn it off when I'm not using it.

Are there any disadvantages to rackspace cloud hosting versus a VPS like slicehost? How complex is it to setup and manage vs. a VPS?

Rackspace Cloud server is basically the same as an ordinary VPS. The only real difference seems to be richer set of options in a control panel - you can resize Rackspace Cloud server with very low downtime, you can do snapshots from a live system without stopping it and you can deploy new servers very fast. Rackspace Cloud server is a Xen-based VPS under the hood.

I use Rackspace Cloud for my server and I can tell you they are a fairly good host (excellent customer support, superb uptime). Setting up a cloud instance with them is a snap, and happens fairly quickly; also, making a backup is very very easy.

However, it's not all fun and games - their control panel is slow, is not quite as powerful as I've found from some other hosts, they do not (yet) have a programmatic DNS api (and they handle DNS slightly oddly) and their out-of-band console is rather weak.

You may be interested at also taking a look at Linode; we use Linode within our company for individual applications (for example, we have one cloud instance for our task management system). They hold up very well, although Linode did have some horrific downtime in the last couple of months (they should've fixed the source of this, though); I discovered that they're pretty slow to update their website when such a crisis happens, but their staff and the community in their IRC channel were great and very helpful. In terms of their control panel, my only primary complaint is a lack of granularity in the bandwidth reporting (though I don't touch the control panel often, so I'm quite possibly using it wrong)

If you're after a performance comparison of various hosts, Eivind Uggedal seems to have done a decent comparison of Slicehost, Linode, Amazon EC2, Rackspace Cloud and PrgMr: http://journal.uggedal.com/vps-performance-comparison