Great 1500 which will be better Ivysaur or Venusaur?

Do Charmeleon, Ivysaur or Metang have a 100 or 110 power OP two bar charge move? There is your answer. Remember Giratina...good stats (and therefore good CP) with bad moves mean nothing.
You could asume same movesets as you say but really the people you will be facing would probably use optimized movesets and you would put yourself at a disadvantage using middle evos

Assuming the same moveset?

Then ivysaur would be nearly identical to Venusaur. Probably same goes for Metang and Wartortle.

Charmeleon and Dragonair have an advantage over their final evolutions as they're not hindered by the flying type being added.

Pupitar is probably at a disadvantage not having Dark typing and having ground instead (I think Pupitar = Larvitar, not Tyranitar for typing).

However - none of this matters.

I considered getting a 1500 dragonair instead of Dragonite. Except while the typing is superior, the moveset is way worse. Dragon Breath < Dragon Tail. Dragon Pulse < Outrage. Both fast and charge moves are much weaker.

For The starters + Metagross: missing the signature move is a huge letdown.

Already figured Tyranitar is better than Pupitar.

So none are useful.

I checked it out a little bit and they'll be about the same. What you then need to consider is, as stated above, typing and moveset.

CD Venusaur gets a far better Charged Move at Frenzy Plant, though its dodge window is a bit on the late side. Ivysaur also has a strong 2-bar move in Power Whip with a nice early window but with far less damage. Stick with Venusaur if you can evolve a low level one.

I wrote a rather long post to address this question here: https://pokemongo.gamepress.gg/q-a/pvp-and-community-weekend

However, the short answer is that when CP is capped, three things matter: stat shape, quality of attack moves, and weaknesses/resistances.

For this weekend, we need to look at what pokemon have good attacks from their special moves (all gen1 starters, Larvitar, and especially Beldum). So it makes sense to target evolving some of those (during this very limited window of opportunity) to get those special moves on Pokemon with CP < 1500 and CP < 2500. I have a pair of Meganium with Frenzy Plant and CP 1490 and 1491. Note that IVs don't matter. Let me repeat that, just in case it got filtered by your brain or something else in the info chain.

FOR CP 1500 AND CP 2500 LEAGUE BATTLES, INDIVIDUAL STAT VALUES (IVs) DON'T MATTER.

Feel free to take something with decent or even below average values. I have a Meganium, 4/9/4 (ads), Vine Whip and Frenzy Plant, CP 1490. It will fight just as well as a perfect, 15/15/15 at level 21.5, which will have CP 1480. Sure, mine is level 24, that doesn't really matter - I caught it at level 24. The significant difference is that mine will have a bit less Attack and Stamina (not 11 less, maybe 1-3 less) and a bit more Defense.

Once we know more about the specifics of PvP, we can decide what the best possible pokemon to use are. If those pokemon turn out to be starters and other pokemon with CD special moves, this is when we can get them. If it turns out Crobat and Claydol are best, we can evolve and power those up once we know what works best. To prepare for the possibility that starters, etc. are best, during this window power up CP<1500 and CP<2500 versions of Venusaur, Blastoise, Charizard, Beldum, and possibly Tyranitar.