I have the same little lock box in my car. It works very nicely, and I've never had it slide out from under my seat. The cable is pretty heavy, tin snips wouldn't do the trick. I'm sure bolt cutters would, but most crooks don't carry them around.
My car gun is a S&W 4046. It was very inexpensive, so on the off chance it gets stolen, it won't break the bank. It is, however, a full-size handgun that is easy to shoot well, very accurate, holds a reasonable amount of ammunition fully loaded (12 rounds), has night sights, and is totally reliable. I keep a holster for it in the glove box, just in case it needs to become a impromptu carry gun. The reason I chose this gun is the magazine disconnect. I can take the magazines with me when I leave the gun in the car. Then, even if someone does steal it, they are going to have to come up with a S&W .40 magazine before they can shoot it.
My car is not exactly an appealing target for thieves anyway, I don't leave valuable items sitting out in plain view, I always lock my doors, and I have a factory stereo. The huge majority of crooks are going to pass it by for those reasons anyway. It could still be broken into of course, but I think its worth the risk to always have a full-size handgun available.