Well the sg05 with 450w psu here costs 150 odd? The ps08 with 500w essentials costs around 100. Itx boards are more expensive, and you can get an m atx board cheaper. Plus if you need an optical drive it's also cheaper for a standard one vs a slim with required adapter.
This all assumes you're short on cash. If you're not, go for the small form factor, I prefer them, but they tend to cost more, and will always cost more to upgrade as well typically.