Both make a large range of machines. IBM make "servers" all the way through to mainframe-size systems.
Personally I do not like Dell - one of my customers bought a rather expensive Dell server a few years ago; it looked inside like an economy desktop, nothing in any way special, just with an alternate BIOS and a server version of windows with it..
IBM are not cheap but the hardware tends to be good.
Note that a "server" is more a software function than anything, unless you need a special case to fit in a rack with other servers.
Any good quality high end PC can have a server operating system installed on it, or just a standard system if that has the the server components you need already.
We just build machines similar to high-end gaming ones, but without a fancy graphics card, then install Centos linux. That has server components to do just about anything you can think of and is free..
In fact for many things you don't even need a high performance machine, as long as it has good disc drives; I think that's why the Dell one we replaced was so minimal.