I had the exact same dilemma not too long ago, R2 Sunspot vs R5 Domino. Ended up going with the Domino first (so I could take a run through LoL), but Sunspot will be going up soon, as I need him for a few paths in 6.2 and 6.3.
I really enjoy his playstyle, arguably more so than Domino's . Contrary to popular belief, he's very good against stun immune/unevadable specials matchups, as he can perfect block entire combos and specials, purifying incinerates by doing so - essentially achieving the same effect as charging a heavy, without having to rely on parry-stunning. This aspect of his abilities seems to get overlooked often for some reason.
Also a good prestige investment down the line, if you happen to dupe him.
tldr: many reasons to rank either, priorities the one you need for the content you're next going for, rank both eventually.
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I really enjoy his playstyle, arguably more so than Domino's . Contrary to popular belief, he's very good against stun immune/unevadable specials matchups, as he can perfect block entire combos and specials, purifying incinerates by doing so - essentially achieving the same effect as charging a heavy, without having to rely on parry-stunning. This aspect of his abilities seems to get overlooked often for some reason.
Also a good prestige investment down the line, if you happen to dupe him.
tldr: many reasons to rank either, priorities the one you need for the content you're next going for, rank both eventually.