They're both PDE-5 inhibitors but the structural formula differs.
Both can cause stuffy nose but as with many drugs the individual factor comes in. For some the side effects are harder than for others.
I haven't personally tried Cialis (Tadalafil) yet but I might request a prescription for it as I have an appointment coming up.
For me Viagra (Sildenafil) causes very slight stuffy nose if at all. Usually it gives me slight "runs", but not in a way that I'd consider quitting it, it's a timing thing.
In that sense I am curious of Cialis as it should remain active in your system for a longer time.
For now I'm doing more of a herbal route though, I ordered some Butea Superba powder and I actually capsuled some a week ago.
Too bad about Brexit, I had just found an online supplier from UK that had decent pricing for empty capsules. :D Well, my currect stash will last for a moment, hopefully I'll be able to order from that supplier again.
I do search for suppliers in EU as in to get stuff quicker without the worry about customs and taxes but for certain things I have to order abroad. (Sometimes the price difference is too high in comparison to order from EU and abroad.) Whoops, off-topic! :D