Mini ITX vs Micro ATX

I generally recommend micro-ATX. Mini-ITX boards are far less capable and come at a price premium

Usually the MINIs come with more PCI or PCIE ports than the Micros so you should make your selection according to your hardware needs. If you have enough ports to support all your hardware or if it makes no difference to you, then just follow your budget!

A mini-ITX motherboard is just as capable as a micro-ATX board. The principal difference is the number of expansion slots. If you're building something that doesn't need more than 1 expansion card then mini-ITX will be fine.

Smaller does not mean cheaper as you've already found out if you look at the price difference between your 2120T and a 2120. Most mini-ITX motherboards tend to be more expensive than the micro-ATX counterpart.

Without knowing anything about what you plan to do with this system   (hint, hint). I would match the 2120T with an ASUS P8H61-I motherboard, one or two 4GB Kingston low-profile DDR3 1333 memory sticks, a 128GB SSD and one of these cases (which depends on if you need an ODD or not):

http://www.ecosmartpc.com/ei5.html
http://www.ecosmartpc.com/ei7.html

If you want to use more "regular sized" parts (PSU, ODD, 3.5" HDD) the Lian-Li PC-Q07 or PC-Q11 are nice mini-tower PC cases that take a mini-ITX motherboard.

You have it backwards. Here is a normal mini board- http://www.newegg.com/Product/ProductList.aspx?Submit=E... --> far less expansion slots than the micro boards- http://www.newegg.com/Product/ProductList.aspx?Submit=E... which are much more abundant and I think a better choice unless you are building an htpc.
Here are smaller cases built to fit those boards- http://www.newegg.com/Product/ProductList.aspx?Submit=E...

Yup you're right, the other way around!