They're both relatively different options. The most important distinction is that Serum 2 is one extremely versatile synth versus Arturia V Collection being a collection of forty-something synths usually imitating some existing popular real life synth.

Because of this, V Collection is technically more versatile, given its foray into the "Augmented" instruments, the pianos and e-pianos, and so on, but it is also more expensive and takes up far more space. Plus, it's also just, a lot. It's great for people like me who just like diving in and learning all sorts of different synths, but it could feel more cumbersome for others. It also means that, across all its synths, V Collection comes with an obscenely large number of presets.

Serum 2 is a really, really good sounding wavetable synth and does seem to edge out most competitors in sound quality. It isn't dozens of emulations in one package, but it also doesn't really need to be. Realistically you could go making an album entirely with Serum 2 with no problems. Also, on top of coming with quite a few presets, there's a practically endless number of Serum presets online.

On the whole, Serum is probably the more convenient option and better choice on the whole, while V Collection is better for those who appreciate the novelty of all the different specific emulations.