So I did some research (MtG used to be one of my nerd-life's biggest obsessions) last night.
For starters I consider Magic Duels superior to Hearthstone - its closest direct equivalent and rival - for a simple reason: It's far more affordable. Mathematically speaking if you buy (be it with in-game earnable gold or real life $$) 72 boosters on M:D you are done - you have all the cards unlocked at the cap, and you only need to do it again when new expansions hit every few months. With Hearthstone there are a lot less ways to keep up or, worse, catch up to other people who've been playing a while. Someone else did the math and a new player starting today is around $300 on average behind someone who started a year ago and has been doing dailies, with the gap only growing. Furthermore, on Hearthstone you need to find enough of each card that you want for decks - the expense is significant. Hearthstone does look damn good though, it's a very polished product (Blizzard is a little better at making video games than WotC, eh?).
As for MTGO vs M:D, the former is of course the most robust product. It features the full card list for many, many expansions as opposed to 80% of each of a few recent ones and utilizes all of Magic's rules rather than a (slightly) simplified version. It, however, is a reflection of the paper game - you need to buy a lot more boosters than 72 to keep up, although of course being able to trade helps in focusing on some specific decks or colors. On the other hand it looks significantly worse and the cumulative complexity of supporting a decade+ worth of mechanics has cause more than a few technical issues and bugs. On the other hand MTGO does give players the ability to be part of sealed deck tournaments, participate in real ladder play, etc. So for competitive players it really is the only way to go.
To me at the moment M:D looks like the superior game. It's slimmed down but it's manageable in terms of how much I'm willing to spend playing online. I mean I have 1400+ cards on MTGO from back in the day but almost none of them are Standard-legal any more, and I don't look forward to a huge investment just to start playing.
I did install both Cockatrice and XMage yesterday. Both of these have a main advantage - since they're unofficial projects you get the whole database of every card and they are all free.
Cockatrice looks good! It's very clear in what it does and the deck-building interface is pretty sleek but I didn't know anyone to try and play a couple of games to see how it handles itself. Basically it's a virtual table; you tap, keep track of life totals, make sure your deck is format-legal etc all on your own just like the paper game, so I read this sometimes causes complications with cards which have weird flip-over effects and whatnot.
XMage is written in Java and looks... messy, at least to me. YMMV. It's supposed to be automated though, so cards actually do whatever they do like on MTGO/M:D. If I find an opponent tonight I'll make a deck there and give it a whirl.
