Regardless, if you want the cheapest, I would go with Digital Ocean, though it is a different kind of hosting. For Rackspace, you really go when you need their support people and not just a server.
I hear a lot more developers this year going to DO than last year, but I have never used them though, trying to stick with the "devil I know."
Digital Ocean looks roughly comparable to Amazon for RAM, and slightly ahead on price-per-core (if you assume all cores are equal), but way behind on SSD. Again, this is assuming you want large instances and are going to run them 24/7 for at least 6 to 12 months -- otherwise, DO pricing starts to look a lot better.
EC2 i2.4xlarge: you get 16vCPU, whereas that Rackspace gives you a DUAL 10 core Xeon E5, 40vCPU - it's 40 cores, not 20.
Memory is much of a muchness.
The drives are 2 1.6TB OCZ PCIe devices that can be RAID 0'ed if you choose, and each has 400,000 IOPS, up to 800,000 combined, whilst the EC2 instance tops out at about 155,000.
For example, compare the "I/O" server with an EC2 i2.4xlarge instance. The Rackspace server has 128GB RAM, 3.2TB disk, and 20 cores; i2.4xlarge has 122GB, 3.2TB, and 16 -- nearly comparable.
On EC2, I can buy a 3-year Light Utilization Reserved Instance for $3884, and then pay $828/month (based on 720 hours per month). After 12 months, my average cost has been $1152/month. I still have two years left on my reservation, which I can keep using, or possibly sell, so the effective cost is even lower.
If I'm more certain of a 12-month server lifetime, I can buy a 1-year Heavy Utilization Reserved Instance for $7280, and then pay $447/month, for a total cost of $1054/month.
On Rackspace, list price is $1800/month. Suppose my total spend is $10,000/month (list price) and I commit to 12 months. I get a 15% discount, or $1530/month. That's quite a bit more expensive than EC2, and with EC2 I'm committing less up front. A longer commitment would help, but it would also bring down the EC2 price.
Can anyone poke holes in my analysis?