Tadalafil / Cialis in 2.5mg is mainly used for prostate problems. It can help with erection but the common dose from my understanding for Tadalafil is 10 mg, 5mg can give a proper enhancement. I have 10mg but I haven't tried it yet. (Considering I got a proper prescription and the price isn't the cheapest, way more expensive than sildenafil, so I'm kind of saving the tadalafil for even more special occasions.)
Sildenafil / Viagra, usually 50 - 100 mg dose is most common. Depending on your health etc., one might get erection enhancement from 12,5 mg dose, that's 50mg split to four pieces. Certain manufacturers make a sildenafil pill that is easy to split. (Longer time ago I had one brand and I did get enough help from the 12,5 to enhance my girth training.)
Depending from where you are, getting a prescription should be relatively easy. And that way it'll be safer to get the real deal.
I would say it's more so about "swallowing your ego" and just asking for it during a regular check up. I find it easiest to explain something about stress. Of course be wary of any underlying conditions; it's a drug / medication after all, not candy. With certain exercises and training you should be able to make your erection very strong and powerful without any medication.
It's good to get a check up etc., and proceed with caution. Overall though sildenafil / tadalafil etc., prescriptions are written off quite easily, at least in my experience.
A TL;DR: Sildenafil for "daily" girth regimes, I'd save Tadalafil for sexy weekends. 
