Y'know what? We haven't got a clue what to advise you -- you haven't given us enough to go on.
Can you tell us the type of work you'd be doing on the Mac, what your budget will stretch to, whether you already have a monitor, and what it is, and whether you already have a mouse and keyboard? Do you need a printer? Software?
Your friend, if they were using a Mini with the stock 256 Mb RAM, may be correct. 256 Mb RAM sucks badly on most programs. You must upgrade a Mini to 512 or 1 Gb. For most people, the best way is to buy the upgrade RAM from a reputable memory dealer who guarantees compatibility with the Mini. The adventurous can open the Mini case with a couple of flexible putty knives. Technophobes can have a technician install the RAM.
Of course the iMac G5 is better. It's faster, has a nice screen, delivers with a keyboard, mouse and 512 Mb RAM stock. However it's also a bunch more money.
However -- I have a Mini as my database server and run another Mini for all my office requirements, including Filemaker, MS Office, Photoshop, Quark Xpress and Illustrator. With 1 Gb RAM it is by far powerful enough for me.