I've been asking myself the question for awhile.
For points, Sapphire is the clear winner. 7% bonus and more flexible (can be converted 1to1 to UA and a few other airlines, Hyatt or used to purchase a ticket (via Chase's portal) on any airline.

For a 1K (and I believe Plat), the annual fee is higher than United Club membership. I don't know what the membership fees are this year yet, but last year the cc fee was $50 higher (I'm grandfathered in at the $375 annual fee). Originally, I looked at the $325 being UC membership as if I had purchased it directly, and the remaining $50 being the cc annual fee .. a decent fee for what used to be a great card. Now that the card's benefits have been reduced, I've obtained the Sapphire and it has become my primary card. Now I question if I can justify the "$50" annual fee.

With the PPlus card's benefit devaluation, and being 1K (which means I get most of the remaining benefits anyhow), I've about decided I don't need the card. The only remaining benefit I can think of that has any use is upgrades (CPU) on award flights. That sounded nifty at first, but I always use my miles for premium International seats. I think the cc's treating awards flight as CPU upgradable means the benefit is limited to domestic flights only.

I don't think "UR" is some fancy FlyerTalk acronym. It is used by Chase themselves, and is the logo for Ultimate Rewards.

Booking flights thru the UR portal will get get you 3 (instead of 2) miles per (any) airline dollar. I definitely use it the few times I'm not booking UA. If you've ever used Expedia, then the interface will be very familiar to you.

A warning when using Chase for booking UA ...
This experience was pre-SHARES integration. I've no idea if SHARES would have the same issue. And since SHARES doesn't show "pending/waitlisted/confirmed" on your CPU requests, there's no way to tell
UA got the reservation within minutes of booking it via Chase, and it did reflect my MPlus number and 1K status. However, once inside the CPU window, my upgrade status did not change from "pending" to "waitlisted" or "confirmed". UA knew I was qualified, otherwise how did my segments show "pending". I manually deleted the upgrade requests, then re-requested them ... all segments then updates to "waitlisted". That was the only time I booked UA (and never CO) via Chase, so don't know if it was a fluke.

My response has been more towards renewing or not. Your question was purchase with PPlus vs Sapphire. But I think the 2 are related. If you think you'll keep the PPlus, then go ahead and use it. But if you plan on dropping it, then I'd say go ahead and start using the Sapphire card.

Me personally, while I've mentioned that I see no true benefit in keeping the card, plan to keep it for now. I plan to retire in a couple of years so don't think I'll keep the 1K. At that point. FEQMs and upgradable award tickets on domestic flights might become useful to me.

I will say that since the benefit reduction, I only use my PPlus card for UA purchases (previously, it was my primary card), so I've reduced the benefit back to UACO, they're selling fewer miles to Chase on my behalf