There is no way to use a real arcade gun, so forget about that. Unless you're technical enough to figure out an interface and driver yourself.
The Act-Labs is currently the ONLY option that plays well. There are a few other products out there, but performance is less-than-optimal. There's some promising development on using the GunCon2 (for Playstation2) with the PC, but it's not working all the way yet.
If you don't care about price, and want the sure thing, order the Act-Labs before the 30th.
If you think the price is outrageous, and think the GunCon thing looks like it will pan out, go with the GunCon.
Or, do nothing and wait to see if something better comes along.
Those are really our only options.