Tough call, but I think the playoff structure of CL makes it fairly easy to re-format it (I know Euros has a similar format, but tinkering with it wouldn't make it much shorter). So I'll cheat and say I'd rather tinker with the CL than the Euros (after all, we've already invested over a year in qualifying for the Euros).
I would damp El/Cl 2021 if I were to sacrifice one. However, I am sure both will take place some time next year.
As a fan of Manchester United and having the only UEFA national team I like not being a contender (I'm not saying who), I'd rather scrap the Euro. There are many people who are fans of a UEFA club but not a UEFA national team. I normally don't like when fans complain that national team games are bad for clubs, and I don't like that national teams have to have youth tournaments and qualifying for them when clubs don't have to release players. Given who I'm a fan of, choosing between national teams and clubs in UEFA is different for me than choosing between every national team and every club. I'm not expecting other fans to agree with me, and I'm not claiming that scrapping the Euro is better for more people.
I would rather scrap Champions League and each league all over Europe have their domestic seasons and then forward on to Euro 2021 instead
Scraping Euro's also means no more qualifiers or friendlies, which seem ridiculous right now. 2020 season starts in sept and there is a int week immediately then 2 more before the end of the year. trying to fit all these games in a new condensed schedule seems ridiculous.