There are advantages and disadvantages, of course. Should you buy the server version? As always, the answer is, "it depends." Do you need to run any of the following?
- Mail server
- Calendar server
- Address book server
- File server
- Podcast production
- Wiki hosting
- Web hosting
- Centralized Spotlight search
- Centralized management
- VPN Server
Most or all of these features are available in free, open source products (in fact, many of them are based on the open source projects). You can add OpenLDAP, OpenVPN, Apache, Samba, Calendar Server, etc. to a client Mac OS X installation if you want. However, Mac OS X Server wraps them up into a nice, aesthetic, easy-to-administer package. This could be a net positive or negative to you depending on what you want from this machine.
By purchasing a Mac mini server, the extra hard drive is nice but you sacrifice the DVD drive. This might be a deal breaker depending on what else you plan to use this home server for. You can always add an external one, but that is extra cost and hassle. Again, it's a trade-off.