I recommend bitminter.com as an easy to use miner and pool to just jump into.

The reason not to mine and spend your money on btc's rather than mining hardware is straightforward. BTC price will in all likely hood only go up from here. With an occasional sell off dropping the price for a few days.
The reason to mine is a philosophical one if your not going to make much doing so. If a single entity gets more then 50% of the hashing power they effectively control the network. So you mining even a tiny bit helps keep that from not happening.

In my opinion. Everyone who owns and uses bitcoins, should have a miner running to support the network.