Reverse holos are holographic versions of common/uncommon/rare/rare holo cards that can be found in the set. As of the 'Legend Maker' set, there are no reverse holos of PokemonEX, secret holo rares or box topper cards. In other words, you'll never get a reverse holo of CharizardEX.
Generally, EX cards(I've never seen a reverse holo for a card with "EX" in their name...like Arcanine EX) are more rare than those that do not feature "ex" in the card name. The ratio of rariety depends on the set and the amount of EX cards in that set.
As someone who specifically collects "specimens" instead of plays with them, I generally buy one or two boxes of a set when the cards are released on the market. My experience is that each box in each set contains the same amount of EX cards. Nintendo is pretty good about keeping the amounts to 12 holos/36 pack box, though, I find that the ratio of holos/pack is higher in the initial first week of the release of the set than it is at any other time.
Anywho, getting back to the rariety of the cards, the rariety of the PokemonEX card depends on the set. Most of the EX sets feature two Pokemon EX cards in each box. There are two exceptions to this rule - "Unseen Forces"(which included 3 due to the amount of EX cards in that set) and "Delta Species"(which included 1 EX card as the set only consist of 3 Pokemon EX cards). So, if you have a choice...go for an Unseen Forces box.
So, to get back to your question, a "Pokemon EX" card is more rare than the standard holo set(with exception to the Shiny Pokemon....they're 1 per 3 boxes), so, your Deoxys EX(assuming it's the one with the silver border) is more rare than a regular Manectic holo card.
