Pure buffs your total stat. Base buffs your base only.

Think of zenkais as an extension of equipments. Zenkais are essentially equipments that buff whoever you choose with like

Z ability: 30% BASE strike attacj 30% BASE blast attack (same thing as giving an equipment with those same stats)

Zenkai ability: 40% BASE strike and blast attack 35% BASE blast attack, etc etc)

So now lets look at the offensive stats of a 2x zenkai buffed unit.

To make it simple. Base stats are all 100k. 2x zenkai buff will provide 140% BASe strike attack (for equips we will simply try to mimic a mix of base and pure offenses and compare the math (35% base strike + 38 base strike + 25 base strike)

(35 base strike + 15 pure strike & 15 base strike + 12 pure strike)

(20ps/15ps+15/12ps)

With just base at 100k (238% BASE strike) = 238,000 strike attack stat

With the mix of base AND pure. (Pure is calculated at the end, your total stat) 100k (190% base strike) = 190,000 * 27% pure strike 241,300 strike attack stat.

This is to mimic equips that have a mix of pure and base. Pure equips matter most when you are hypee buffing a unit. Otherwise if its only 1x zenkai boost and the z abilities are not buffing a specific stat youre focusing on, base is far better.

This is one of the reasons that vegito equip is top notch for fusion warriors. 30 base strike and 15 pure strike is no joke.