Being in the Cleveland area the last 4 years or so, I've seen a lot of Irving, including the last season before James. The only person that can consistently stop Irving from getting off whatever shot he wants, is Irving. What I mean is, he is pretty reckless with the ball. He commits a lot of stupid turnovers (things like bouncing the ball off his foot). He is perhaps the greatest ball handler the league has ever seen. He ranks right up there with guys like Iverson in that regard, however unlike Iverson, he is an excellent shooter from every where on the court. Although also unlike Iverson, he doesn't like contact, so he doesn't get to the line as much as you would expect for someone who drives as much as he does. Tying into that recklessness is that Irving quite simply is not a very good decision maker or playmaker. While no one can consistently stop him from getting to where he wants to go on the court, he often makes the wrong decision once he gets there. He also doesn't have great vision and he is an average at best passer (for a PG). One of the reasons the Cavs are so bad with Irving and without James on the floor is because Irving just isn't a great playmaker (while James is one of the best ever).
That said, much of that also very much describes Thomas, however as a result of Thomas' size, he can be thwarted a lot easier than Irving can, especially against the better teams that you play multiple times in a row in the playoffs. Also, while Irving rarely shows any interest in playing defense, when he tries he can at least be an average defender (he clearly doesn't try as much as he should). Thomas tries a lot more defensively, but his size is a definite impediment and that will always limit his defensive effectiveness.
When you couple in that Irving's contract has the ability for Boston to avoid the tax in the 18-19 season, he is younger, and has more defensive potential, I think you make the trade if you are Boston. What I don't think you do is give up Thomas and a prime pick. I think something like Thomas, Crowder, and Rozier for Irving makes sense for both teams and would generally be a fair trade. If they wanted Boston's 2018 1st, I wouldn't make that a deal breaker either, but given the Cavs shave 3.8 million from their payroll (which is like 10 million with the tax), I think they should be ok without the 1st, especially if it leads to them dumping someone like Felder for a protected 2nd round pick or something (to save them even more money).