I don't think it's very fair to compare Porygon-Z and Keldeo; while Keldeo does have the luxury of being able to take down Chansey relatively easily, I find that I usually end up running two or three checks to him if only because Latias, Florges, Roserade, Jellicent, Vest Slowbro, Celebi, Toxicroak are decent to amazing in this metagame. Granted some are shaky checks at best and I mean I'll be the first to admit Specs Keldeo puts a ton of pressure on teams, but the metagame shifts balanced it fairly well.
252+ SpA Choice Specs Exploud Boomburst vs. 252 HP / 0 SpD Florges: 204-240 (56.6 - 66.6%)
252+ SpA Choice Specs Adaptability Porygon-Z Tri Attack vs. 252 HP / 0 SpD Florges: 204-240 (56.6 - 66.6%)
Porygon-Z and Exploud actually have identical damage outputs, with the main difference between them being one of bulk vs speed. I myself would always pick P-Z because Exploud's bulk, even invested, does not help it any against revenge-killers, and out of the two he is actually the one who is hard walled by Chansey. Consider this :
252+ SpA Choice Specs Exploud Focus Blast vs. 252 HP / 4 SpD Eviolite Chansey: 216-256 (30.6 - 36.3%)
Meanwhile P-Z has access to Trick - people are discussing Knock Off to death but that doesn't mean it's the only move Chansey has to deal with :p
Exploud's bigger claim to fame, of course, is being able to spam Boomburst without fearing Ghost-type switch-ins, but it's not like Porygon-Z is helpless against them either. The Dark-type buff was actually very kind to it as it has access to perfect coverage in just Tri Attack and Dark Pulse. A bulky Agility / Nasty Plot set (with like 176 Spd / 252 SAtk / 80 HP EVs to outspeed Scarf Mienshao after an Agility) is very viable and punishes both frail offensive teams and Bulky Offense not running Chansey. All in all I think out of the two of them, Exploud will be the one to drop if only because of his inferior speed.
(Not to mention you could just run Double-Edge)