Coolant alone will lubricate and protect the cooling system components better, but pure water is actually the best coolant as far as taking heat away from the engine and expelling it via the radiator. Thus, cars use 50/50 mix to get the best of both.
The Pentosin pentofrost will run you $27.70 for a full gallon if you purchase at Advanced Auto.
I choose to run Zerex G-05 from Walmart. Comparatively, a full gallon is $13.88 and it has the HOAT chemistry which is highly recommended for Volvo and I think it's the only real thing that makes the OEM coolant special.
Zerex G-05 here, as it's readily available almost everywhere should I need coolant in an emergency.
Whichever coolant you run, make sure you flush very thoroughly and dilute down to 50/50 with distilled water. Minerals from tap water or additive packages from different coolants mixing are bad news bears. I'd argue that this is more important than the type of coolant being used.
Normally, the coolant does not need to be changed. If the system must be drained, consult your Volvo retailer.
NOTE: Do not top off with water only. This reduces the rust-protective and antifreeze qualities of the coolant and has a lower boiling point. It can also cause damage to the cooling system if it should freeze. Top off with Volvo Genuine Coolant/Antifreeze only (a 50/ 50 mix of water and antifreeze)."
I disagree with the last comment on that other thread, don't pass anything but distilled water through your system at any point. All the minerals and chemicals in city or well water are going to love chemically binding with a lot of the surface area in your cooling system and then contaminate the fresh coolant you put in right after.
There is a reason you only want coolant and distilled water in there after all.