Amex bonuses are lifetime now, so this would be the only MR bonus you'd be eligible on the regular Platinum card, ever. Every now and then a 100,000 MR bonus slips out (and usually with only the $3K min spend), so you should probably wait for that if you don't think you can hit the min spend for the 50K points. I got the Mercedes Benz platinum for the lifetime cap alone when they had the 75K bonus points offer. The $400 airline airfare credit is an interesting bonus, so you'll have to decide if the fewer points equal the $400 in credits. The 25K bonus is not worth it at all in my book, since you'd potentially be missing out on 75K points if/when another 100K offer drops.
The Points Guy had a breakdown of different MR redemption options
, with one option of transferring 30,000 MR points for a round trip flight to Hawaii from anywhere in the US. If you're on the East Cost, that could be a large savings. More than $400, but would also require waiting for a 100K bonus to show, you hitting the min spend and transferring and redeeming the points. My hope is it's still an option next year when I fly for my honeymoon.
Given the lifetime cap, I wouldn't "try out" a platinum card without a high bonus offer (75-100,000 MR), not to mention the annual fee.