Viagra is a brand of pills, there are not different "types". There are just different drug doses depending on how much of the drug you need. Viagra is prescribed by a doctor to men who have erectile dysfunction, not those who have a little trouble keeping it up sometimes. Are you over the age of 65, and do you have problems actually getting an erection when you are aroused? If not then you probably arent a good candidate for viagra.
Try a penis ring instead. You can pick them up at any sex store. A penis ring is slipped on when you are fully erected, and traps the blood in the penis, keeping it hard. Just be sure not to leave it on for more than 20min, or it can damage some tissue.
You are not to stimulate you body but to make it healthier, not to crack a whip but to give strength. Use natural things, for example zupro to give a chance to your organism to make right decisions and to behave itself.