Delta pokemon aren't even legal anymore, but anyway...
Dlta pokemon don't in any way compare to lv x's or exs just ordinary pokemon that have either a metal bonus oa chnaged energy type. Personally I thought the whole delta era was bad because the only dekc people played were holon engines... and lets face it, they were amazing. It was annoying how little variety of decks were produced, and playing holon engines over and over is really boring becasue no one used anything else.
I prefer exs to level xs as for a level x you have to sort pf evolve the pokemon again, making them slow to get out. lv xs aren't that much better than some normal cards anyway, although some like the inferanpe, torretera and empoleon ones, were very good, lots were not much better than normal pokemons, ie lucario. Exs are muhc easier to use, and generally stronger.
THe only downside to the ex is the 2 prize penalty. That is probably why lots of people prefer lv xs.
just my 2 cents.