Does anyone here know whether there are differences in performance for rackmountable servers and mid-tower servers ?

The only difference is the case. If the case is too small, you might encounter heat problem.

Just the size of the case.

There is nothing you can do in a tower case that can't be done in a rackmount case.

When colocating a server you will find that you will save money by having a rackmount case because of the less space needed to store it.

As for the cooling they have 1u and 2u cases that cool better than most tower cases.

One other bit of info:
I would consider a "midtower case" to be at least equal to a 2u server -- as you can cram at least 4 hard drives in a Midtower - when most 1u cases only hold 2 hard drives....

most mid towers use 4u

it is not worth using a mid tower and not going rackmount

There are 1U servers with space for 3 hard drives out there. The most important thing about 1U servers/cases is the cooling ability of the server/case. If you get a 1U (especially) make sure it has adequate the cooling capability.

As a few other people who responded mentioned, the biggest difference is in what you can fit in the case and in cooling. I've seen one server production company (that is, they sell hardware, not hosting) that sells a configuration which permits you to stuff four servers into a single 1u case.

Another consideration, if you are going for budget hosting, is cost--both start-up and monthly. A mid-tower case can be purchased for as little as $34, while a 1u rackmount case usually starts at around $230, though I've seen some that are cheaper. Depending on how your webhost charges, you may find that one or the other is a better idea.

I have servers of all configurations in my datacenter--most are mid-tower cases because that's what my clients prefer. Based on this, I set my base-rate to conform with a standard mid-tower case.... bigger than that and there is an additional charge, whether it is rackmount or not. Smaller, and the price stays the same because it's just our standard rate. Some other hosts charge by the unit of space on a rack, others have different pricing for tower cases and rackmount servers.

I think it probably has a lot to do with how the hosts in question are configured. If you are dealing with a host that has a rack or cage in a large colo facility, they're probably going to be more concerned about the space usage, whereas if you are dealing with a host that has their own datacenter with an excess of space, it may not be as much of an issue.

--Erika