By MileagePlus, I'm assuming you meant the United MileagePlus Club card? I don't have this card, so I can't really comment.
@Calfan has posted about the Club card previously:
https://www.disboards.com/threads/i-love-credit-cards-so-much.3528202/page-2323#post-59681667
I think the Explorer is a good United card to get.
Between whether to get a United co-branded card for the miles or another Chase (Ink) card, you can look at 1 UR = 1 UA mile because UR transfers to UA 1:1, but I hate the idea of transferring UR to UA unless absolutely necessary because UR is a lot more versatile than transferring to UA, and sometimes paying for cash seats through the Chase travel portal with UR is a better use of (less) points than redeeming UA miles for award seats.
The fact that the Explorer card often comes with its first year AF waived -- including this 65k offer -- is a real plus, because you can product change the Explorer card to a no fee United MileagePlus rewards credit card after your first year is up.
Whether the 65k bonus is a good one is debatable, because the standard offer was 40k/$3k/3 months, but United often had targeted offers for 50k and 60k (and last year, up to 70k) for only $3k MSR. I think about two weeks ago, United upped its public offer to 50k/$3k/3 months. This 65k offer is a tiered bonus, with 40k after $2k, and another 25k after you hit $10k in 6 months. So that's quite a bit more spend ($7k) for those extra 15k miles.
As for Club access, the Explorer card gets you 2 complimentary passes, but consider you also have Priority Pass lounge access on your CSR.
If you're also looking at the United Biz card, see if you're targeted for the 75k + AF waived offer:
https://www.doctorofcredit.com/targeted-chase-united-business-explorer-75000-miles-af-waived/