You should have gotten PS+ when this game was free.
The boss fights suck but are bearable if you don't mind wasting a few points on combat upgrades, and personally I thought the DLC was a letdown. If they're still making you lose your upgrades for the duration of it, I don't think integrating it into the full game is a big deal, and if anything, it'll just slow the pace down.
If you have money to spare, then sure - go for the improved version. Personally, I'd just go with the cheap option. :p