There's not enough information out to really make a determination. You'd expect 2K18 to be better than 2K17 -- 17 had a rough launch and the multitude of patches it got frustrated many of the player base.
2K18 may suffer from a similar issue, but I would expect it to be smoother.
As far as significant differences go between the two, one immediately comes to mind: jersey patches. Some teams in the NBA will have small jersey patches next season for advertisers. Personally, I HATE it. The lack of sponsors on sports jerseys here in America has been great -- I don't want my teams looking like NASCAR racing suits in a few years.
The NBA will be doing this in limited form till 2020, but many sources believe that it'll expand far beyond that and look much like the Euro sports leagues from that point on.
It's a minor thing and 2K may (wisely) leave it out of the game, because most teams won't be selling retail jerseys with those ads on it. But 2K may not leave it out because they strive to capture IRL NBA stuff.
This article is a good read on it if you're interested.
https://www.racked.com/2017/6/15/156...ba-jerseys-ads
Personally, I'd wait and see what 2K18 has to offer -- it's hitting in September, so more information should be available by August. At that point, the preorders will still be the same price and 2K17 might be cheaper than it is now. It also depends on your play-preference; if you're all about online, than you'll probably want 2K18, as (theoretically) the servers *might* be better.
If you're an offline MyLeague guy, 2K17 has some fantastic rosters for all systems as well as draft classes. Guys really went above and beyond this year with the tools they had (and 2K gave them some pretty good tools). I'd recommend sticking with last year's version if that's the case, at least if you're really into custom rosters.