Should I Power Up This Venusaur?

wait until the event ends...but it is one of the best contenders. you can leave it as is and use it as for training or power it up to use as an attacker

This moveset is very good for Venusaur -- Petal Blizzard may not be as devastating as Solar Beam, but it is significantly quicker than SB so you'll take less damage while delivering it.

I personally would leave this Venusaur as it is because it's perfect for training against the high-level Rhydon and Vaporeon that we're seeing more and more of in gyms, but it's also worth powering up if that's your preference.

I would power up the Venusaur personally (after event)...If you get another later with SB, power that one up too as gyms are stacked with Vaporeons all the time.

1455 CP is a bit too high to get 1000 prestige each time against the bottom defenders and at 93%, it's worth it IMO. If it was at 1200CP, you may want to save it, but really, any non high IV mon can be "saved" for prestiging while a 93% is much more rare to begin with...

You also have the Tangela already and it's natural max CP is lower so it's generally better for prestiging IMO than a Venusaur.

I hope to get one finally with good IVs after this event as I still use Jolteon to attack into Vaporeons now.