As in the above posts carrying a pistol as a secondary isn't entirely ideal, the two reasons for using a sidearm secondary are failure of your main marker or being out of paint.
Instead maintain your marker and practice loading while firing so you are always ready. Both are lighter and easier to manage then carrying another marker.
But practicality isn't always the answer, and there are more pistol / pump side games at events, and its fun.
With pistols it depends on what you want to do with it.
As a sidearm used as a secondary a TPX or Tiberius are pricey for the occasional shot.
Other cheaper markers are availalble in designs such as the Delta 68, which is a Spyder based design and pretty reliable. (The only issue is blowing the 12 gram o-ring if you don't lube or release the 12 gram while it still has too much co2)
But to go on to the two main pistols of the day:
Tiberius & TPX
If firing a small number of shots (up to 24) then take your pick
If putting in more use and sticking to co2 then magazines and 12 grams are the main factor.
Tiberius
The Tiberius allows you to use as many magazines as you like (I say minimum practical is 3) but you have to have a 12 gram ready in each magazine.
The advantage of this is I fire off 8 shots, change magazine and I'm ready to fire again
I now have one full magazine spare and one part used magazine, I click the tab on the bottom so I know it has been used once
If I'm clear to do so then I reload paint
When magazine 2 has been fired reload the next (Hopefully the original, and keep magazine 3 unused)
If I use magazine 3 and have not reloaded magazines then I could be in trouble in a minute (But thats pistol play)
What I aim to do is reload magazines 1 and 2 until one has been used 3 times (using the clicker to confirm) then I change CO2 and hopefully have magazine 3 full still
With 3 magazines the Tiberius has 72 shots if I don't load another 12 gram
The disadvantage is if I do not use the pistol all day then I have still punctured 3 x 12 grams
TPX
What I don't like on the TPX with CO2 is the 12 gram is in the pistol and not the magazine
I dont have to puncture a single 12 gram until I make the first shot, but I cannot reload the 12 gram until it is finished. If have fired 20 shots then the opposition can come at me and I will run out of CO2and cannot quick reload.
Tiberius on remote line air
I prefer to use air as then I am not always using up 12 grams
The problem for the Tiberius is the adaptor goes in the magazine so now I cannot carry 3 magazines and have to reload the magazine every 8 shots
TPX on remote line air
This is my preference
Now I can carry as many magazines as I like and keep switching magazines and reloading the empties when I have a moment
A small advantage is you can attach a remote line to charge the air then remove the remote and still shoot - I have not tested this on a practical basis as to how many shots you get