Nelson and Dellavedova aren't remotely similar players..
Delly is a lockdown defender, but not a very good point guard. He's not a good shooter from midrange, but has a good 3 pointer. He's also got a limited offensive skillset.
Nelson is not a lockdown defender. But he is a fairly solid facilitator. He's got a decent, yet very streaky, midrange game. He's also historically decent from 3. When he's hot, he can be an effective option on offense in multiple ways (not just from 3).
Neither guy is a worldbeater, and my preference is to keep Delly. I see no reason to add Nelson that stands out enough to warrant trading for him; let alone using a roster spot.
But the two players are not interchangeable. They provide very different skills and functions to the Cavs.
For example, you can play Nelson next to Irving and James and have Nelson run the point effectively (better than either Irving or James). Not so much with Delly.
So the idea might be to add a facilitator to the Cavs backcourt who can come off the bench when needed, and that allows Irving to continue playing off-the-ball. Also it might be yet another PG to remind LeBron he isn't best served as the Cavaliers point guard.
I'm glad that we agree on the conclusion which is that Nelson isn't an upgrade over Delly.
Although it's funny that I'm not sure we arrive at the same conclusion in the same ways.
For example, I think you might have (accidentally?) reversed each player: Opposite of what you wrote, Nelson is a slightly better defender than Delly; Delly is a slighter better offensive player than Nelson.
It's also possible that you are remembering the Jameer of year's past, not the Jameer of today.
[Of course, this is quibbling since we end up at the same place.]
But just to show it to others -- not you -- let's use this as a point of comparison. When we break it down, others will see just how trivial the difference is between them this year:
DEFENSE:
First of all, Delly isn't a lockdown defender (man, I wish). He's actually a slightly
worse defender than Nelson, though I wonder if it reaches statistical significance.
Delly's Defensive Win Shares is .2 (that's not good, and a FAR cry from "lockdown defender.")
Nelson's Defensive Win Shares is .6 (that's also not good but actually a tad better than Delly.)
If you prefer adjusted plus/minus to box score measures:
Delly's Defensive Plus/Minus is -2.1 (that's not good at all.)
Nelson's Defensive Plus/Minus is -1.4 (that's a tiny bit better but negligible.)
The are about the same at defense.
OFFENSE:
First of all, in general, Delly is actually an overall marginally BETTER offensive player than the Jameer of today.
Delly's Offensive Win Shares is .6
Nelson's Offensive Win Shares is .0
Two point Mid-Range shooting (you didn't define, so let's break it down between 5-20 feet):
Delly: 5-9ft: 29% 10-14ft: 38% 15-19ft: 20%
Nelson: 5-9ft: 25% 10-14ft: 33% 15-19ft: 26%
Delly is actually a BETTER mid-range shooter than Nelson at 2 out of 3 mid-range areas.
Three point shooting:
Delly is hitting .402 on the season.
Nelson is at .339 this year.
As far as "facilitating" goes:
Jameer has slightly more assists per 36 than Delly but he also has more turnovers per 36, 2.7 vs 1.6.
Delly and Nelson's eFG% is nearly identical.
Conclusion: Nelson and Delly are remarkably similar players at this point in their respective careers. There's really no statistical argument for preferring one over the other.
However:
One
MIGHT try to argue that Nelson has playoff experience, which Delly does not.
To the extent that we find that valuable, or needed, on this team, that
could be a benefit. About the only potential one that I can see.
Of course, it comes with certain downsides, including that Nelson's usage rate is higher than Delly's.
You could argue that Nelson will decide that his playoff experience
entitles him to do more things with the ball during playoff games. Sometimes these dudes have that psychology ("I've been here, I know what to do, you don't, give me the ball"), so that would have to be checked out.
Obviously, we don't want either one of them using the ball too much as there are much better players that ought to have it.