Matchup against the gyms.
Swampert has an incredible matchup against Hoenn’s gyms. I’ll take you through them:
Roxanne: Mudkip wins, Torchic doesn’t do well
Brawly: Both neutral.
Wattson: Marshtomp is immune to electric and is super effective, Combusken is neutral.
Flannery: Marshtomp wins, Fire resists Combusken’s Fire and fighting is neutral
Norman: Combusken’s fighting is super effective, and Marshtomp can mess with Truant by using Dig
Winona: Blaziken is weak to flying and flying is immune to Swampert’s ground, but water is still neutral
Liza and Tate: Swampert’s water beats Lunatone and Solrock, and Blaziken is weak to psychic.
Wallace: Water beats Blaziken and is resistant to Swampert’s water, but ground is neutral.
Blaziken does pretty well against the elite 4 and champion, beating Sidney, Glacia, and Steven, while Swampert is neutral against all but Drayden and beats Steven with ground.
Swampert also learns a ton of HMs- Surf, Waterfall, Dive, Strength, and Rock Smash.
TL;DR: Swampert has a better matchup against the gyms. However, anything works in-game, so Blaziken is a perfectly good choice.
Hope this helps!