If you want the quad core , 1Tb of storage and 4GB RAM now go for it.
You are not forced to use the server app if you don't want too.
Just set it up as you would any standard Lion client.
The real considerations are that the server Mac Mini has the integrated graphics just as the base machine.
The 2.5gb i5 Machine comes with an AMD Radeon HD 6630M 256mb discrete graphic chip for much better graphic performance and allows you full use of that 4GB RAM for running apps and all at a lower cost.
If you can't get that locally buy from Apple on Line