Very Simple.

One of the main differences between the 3G and the 3GS is the faster processor and more memory.

Applications load much faster on the 3GS than on a 3G.

3G is around $99 cheaper than 3GS.

3GS has more battery life.

The "S" actually stands for "Speed".

So, 3GS is significantly much faster.

If you are not worried about Speed, then 3G is okay.