If you’re going to use it all at once, then just buying the bottle with the straw is fine.

However, if you’re going to need it every now and then over a longer period, or you want better flow control and a narrower nozzle, then the pro gun is the way to go. You can (and should) just leave the bottle attached to the gun permanently. It’ll stay good for a few months like that.

If you need to store it longer, just periodically spray some out, then close it up again and it’ll keep fine. In my experience the cleaning solution doesn’t fully clean the gun and it will eventually clog. That said, I just heard one guy say he hasn’t cleaned his gun in 3 years - he just swaps in fresh bottles when they run out and it’s been fine.

The important part is to tighten the flow control knob after use, because simply releasing the trigger won’t fully seal it for long-term storage.