The entire '+' system gives your card an updated effect ( Y-Poola's warp becomes better, cards with 'dodge' chance trigger more often, ect ect. ) or just better stats in general. The sprite and card image thought don't change in reflection towards this, only the larger viewed image. You also unlock that card's CG, that even if there's other versions of them ( Like a few on the starter N card's now have R and SR versions ), they all share that same CG, being the 'same' in context.
As for card suggestions...Hard to say, including that early in. I used Akeno Hibika and Hunter K to 'outpush' crowds as tanky damage walls I could spam, or the team of Virgin Magiress Para LikrA & Magiress Para miiA as they would boost each other and be spamable range units with heals.
...that was of course before the last 2 events that allowed me to get this team up and running thought. The 3 Comets just break everything short that doesn't outright kill them thanks towards their insane attack speed and power ( I've seen one punch a crystal for ~700 damage at only level 35. ) while still being rather bulky in turn. Cacao Cocoa's a 'special' card you can only get from her stage, and you can only play it if you fork over Golden tickets ( I think 10 gives you 20 minutes of playtime on it? ) or spend DMM ( 500 DMM/$5 bucks gives you 2 hours of playtime ), she's great as she's a N range unit who spams her shots at fast speed, applys a slow effect, and is utterly cheap with 'low' recast for range, making her very effective at mass amounts of quick damage and push, and still effective on higher levels thanks towards the slow. Everything else is pretty 'basic' end game team building.
