I recently started playing on Xmage to test out an esper control standard deck that I was thinking about playing on MTGO. My record so far is 20-0 and each game I 2-0ed. When I first started winning I thought maybe I had stumbled onto a great deck, but now I think maybe everyone on xmage is bad and my results are meaningless. I would hate to spend money on a deck on MTGO that I thought was good but actually sucks. Are the player pools of the other mtg alternative clients at least comparable to MTGO or are they all generally new players?
I played against 10 variations of G/R, maybe 5 abzan aggro/control, and 5 random deck brews. I while I played against plenty of thunderbreak regents in mono red, I didn't play against a straight up token aggro red like the one that won the PTDTK. I think that matchup would be the most difficult for a control deck but I can't find any opponents that play that deck.
If you enter tournaments on MTGO you will absolutely play tier 1 and tier 2 decks, piloted by good players. If you are just playing in the tournament practice room, it is kind of a mixed bag. You end up playing a ton of budget decks where people are just testing out ideas. So you may have the same experience on MTGO. I play on MTGO and the only time I end up playing tier 1 decks is when I enter a tournament. You will definitely play better decks on programs where the cards are free, because there are no budget restraints. I've used cockatrice and its a good place to figure out if a deck is legit.