This, right now, is really hard to answer, even in a hedging manner.
Simply, it is too early in the offseason to know these two players' statuses will be come opening day. Gordon was suspended for 10 games this season, and then when he came back the offense looked weaker for it. Meanwhile, Decker had a worse season than his previous two years, and is still with a Jets team that likely hasn't resolved its QB issue.
Lots of things about both of their situations could change rapidly.
Gordon is, I think, the more talented of the two. His 40-time has been clocked as low as the 4.3s, and we only need to look at his 2013 to see what his potential looks like.
This isn't to say Decker is a slouch, at 6'3" with a 40-time in the mid 4.5s, he's a solid receiver.
On the whole though, Gordon is a little more talented.
But then we reach the next question, can he use that talent? That's a much harder question to answer. He certainly didn't this year, not on the field, and certainly not while he was suspended. The offense just didn't seem to work well with him, which is something that could carry over to next season. And that's of course, assuming the team doesn't move him, or he gets re-suspended.
It is entirely possible he recreates his 2013 season, but that's a high bar to reach, especially since the Browns QB situation isn't the most stable and Gordon's success was with Campbell and Weedon, not Hoyer or Manziel.
Still, his 303 yards in 5 games wasn't bad. That translates to about 960 yards over 16 games, and Gordon failed to catch some passes that he caught similar versions of in 2013. If he plays 16 games in 2015, and catches, say, the same % he did in 2013 (with same YPC he had in 2014), he'd just slip over the 1,000 yard mark.
I think that's reasonable, at least for the QB group the Browns have. A tad over 1,000 yards. TDs are tougher to predict. He had none this year in 5 games, but 9 the year before in 14. Split the difference and say 5 on a full season?
Eric Decker meanwhile effectively played a full season (15 games) and ended with 962 yards. Extending that by a game, he too slips just over 1,000 yards.
And he too, has QB problems, that, if the situation holds, aren't likely to be resolved. His TD total this year
So...the answer to your question is that their situations are such that they will probably end up about the same if they play the same number of games.
Gordon has a higher ceiling, but also a lower floor due to his risks. Decker is more stable. Not exactly the answer you probably wanted, but it's the reality of their situations. If you put them on separate teams with clones of Peyton Manning throwing to them as the #1 WR, Gordon wins, but that's unfortunately not the situation.
So I guess wait until you're drafting for fantasy. It's a long way off, but you'll be in a MUCH better position (as will I) to assess if anything major has changed for either of these players that changes their outlook.