In all honesty, the game would have been a 3-4...If I hadn't played the same game in your first version. Other than a new background, a few new interfaces, and some makeup slabbed on the enemies, it's the EXACT same game. Guns are the same, think the upgrades use the same interface from the first one, and it still dies out at the end. As with the first one, once you gotten used to the game, it stops getting harder. Sure, more enemies come and eventually get harder, but you upgrades and gun easily outclass them. There's no real challenge once you get used to the game, and since there are tons of games like this out there, you get comfy pretty quick. It would have been interesting to research gun upgrades, enemies to deal more damage -if you'd put more in- to them, and maybe make it so that the monsters can attack the plant as well. Simply making the plant vulnerable would have made the game more challenging. You'd have to prioritize certain enemies over others rather than just jumping around spamming bullets at whatever was right in front (or beneath because you're jumping) of you. This feels less like 2 and more like 1.1 or 1.2. In the end, it's a game that suffers just like every other game of it's type, longevity. eventually you get all the upgrades, get the biggest gun, and enemies can't touch you while you drop them like god. I actually liked how the ending wasn't what we expected in the first one as well.
In the end, The game is good, runs smooth, and looks nice. I just wish you'd pushed the envelope, or nudged it, or even breathed on it. Essentially, it feels like a new paint job and a new sale sticker were slapped on the original and we were told it was new.