Are strong, slow charge attacks or fast, weak ones better?

I have two Pidgeot. One has a special attack of 25 with 4 blue bars, and the other has a special attack of 60 with one blue bar.

I assume the blue bars are the number of times you can use that attack when it is fully charged, so the 25 deals 100 damage and the other only 60?

Which is better, weaker attacks you can use more often or stronger attacks you can use less often?

Solution 1:

Actually you can use the special attack once a bar is filled. You don't need to wait until all of them are full. Filling more than one means you can use the special attack more than once before having to reload it again.
A longer single bar takes longer to fill than a smaller piece of that bar and you can't fill more of them to save them for later use.

About the damage numbers, welcome to maths 101:
TL;DR: Which Pidgeot to use is completely up to you and your tactics.

The sum of 4 pieces is technically higher, but you have to take the auto attacks into account. When going for a special you lose time for auto attacks, because you spend some time not tapping.

Example:
(this is completely theoretical! These numbers are just for showing how to get the correct numbers.)

Let's say you spend 2 auto attack cooldowns to execute a special attack and you need 4 attacks to fill a piece of special power (and 4 of them to fill the big one). You would then attack 4 times with 25 dmg before attacking once with 60.

Option A: (4x 10) + 25 + (4x 10) + 25 + (4x 10) + 25 + (4x 10) + 25 = 260 dmg
Option B: (4x 10) + (2x 10) + (4x 10) + (2x 10) + (4x 10) + 60* + (4x 10) = 260 dmg

* = Because you don't have cooldown on your auto attacks your special bar will be filled sooner than the 4th of the other one.

Within the same time span both special attacks should even out again on the long term. In quicker fights the weaker one wins and if you can kill your opponent with the 60 dmg attack or within the next 4 auto attacks after using it that one wins.

Note: Again, this is not based on real numbers. It is just an example calculation.