I would like tag dispatch because:
- Easy to extend with new tags
- Easy to use inheritance (example)
- It is fairly common technique in generic programming
It seems tricky to me to add third variant in second example. When you’ll want to add, for example non-POD-of-PODs type you’ll have to replace bool in template <typename T, bool> struct f2; with something other (int if you like =) ) and replace all struct f2<T, bool-value> with struct f2<T, another-type-value>. So that for me the second variant looks hardly extensible. Please correct me if I wrong.