Center tap vs. bridge transformer
The full bridge has the disadvantage that you have two diode voltage drops, which may be up to 2 V. I don't think the cost of the two extra diodes will matter much.

I wouldn't recommend the center tap though: you have a lower voltage drop because only 1 diode conducts at a time, but you'll also use only half of the transformer. So where for the full bridge you could use for instance a 10 VA transformer for the center tap you'd need a 20 VA type.  

The center tap should be avoided as much as possible. If the two diodes voltage drop for the full wave rectifier is a problem, you can probably use Schottky diodes.