Worth investing in for Tempest/General Great league
Answers
I'm assuming you're comparing WG to FB and not IS, right? I would think that ice shard would be better (though legacy), given that it deals SE damage to both ground and flying (while water gun just hits ground for SE) and tears both Tropius and Altaria to pieces. Not sure if frost breath performs as well though, since I seem to remember its DPT/EPT being pretty mediocre in PvP.
For general use WG is better, for the tempest cup FB is the better choice imo. Ice hit for 2xSE damage 2 of the biggest cup threats - Tropius and Altaria, both resist WG and in both cases FB turns a losing matchup to a winning one. FB will also perform better against Abomasnow. WG will win a mirror match and will destroy Charizard, it will also perform better against Skarmory and lanturn.
Don't worry about the 9 attack stat. See where it powers up to. It's possible that the lower attack will allow it to be powered up another half level, giving it better defense and stamina than a perfect one half level lower.
Agree completely that Lapras is best with Ice Shard or Ice Beam. I have one of each, but not one with both (which would be awesome in GL, and really good in UL too, to face Latios and Giratina.) Tempted to take the Ice Beam one that used to be my go to against flying dragons, Frost Breath/Ice Beam, TM it to Water Gun, buy Surf, and power it to just under 2500 for UL. Wish I could TM it to Ice Shard, to play to its strength, but that's legacy.