Hey Kallybear,
I had some sativas indoor and maybe I can help.
The three varieties you mentioned are from Ace seeds and Cannabiogen, not Seedsman. Seedsman only sells them
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So there are quite big differences between these, the OldTimersHaze is a pure sativa with extremely long flowering period and stretch, can go up to 22 weeks of flowering(necessary to put 11/13 schedule with these), while the Bangi haze and Purple Mexican are more of medium length of flower, around 9-12 weeks. PM is a hybrid with PCK so it's the less "sativa" of the three, but can have some nice mexican expressions.
I suggest you go with Bangi Haze and Purple Mexican if it's your first time with sativas, they are more forgiving and grower friendly, OTH is for true sativa lovers with extra patience and skills.
Mind you all, these are very different from the so-called sativas like BlueDream or similar hybrids, not to mention autoflowering so-called sativas. Not comparable at all.
I had a great spectrum of highs from a Congo-dominant sativa while the most commercial sativa Power Plant was a huge disappointment in terms of high.
Now I'm trying to have a hybrid of two superb sativas, Panama and Bangi Haze, it should be a special treat
If there's any more info on these strains you need, just let me know.
Cheers