There is no daily low dose Sildenafil* (Viagra), because it's never been shown to work for penile rehabilitation, and a daily dose can't achieve continuous dosing. To achieve the continuous dosing which daily Tadalafil does, you would have to take Sildenafil 3-4 times a day.
The 5mg daily low dose Tadalafil has been shown to be effective for penile rehabilitation, although the evidence isn't strong. (Some researchers observed it, other researchers didn't.)
*For those taking Sildenafil for pulmonary hypertension (which is what it was originally intended for, and for which continuous dosing is essential), they take a special 20mg tablet 3 times a day. The tablet is a mixture of immediate release and delayed release Sildenafil, to make it last longer than it otherwise would. I imagine it's stonkingly expensive, and it's not used for erectile dysfunction.