A few things. I think you're a bit hung-up on the credit card part when you should be focusing on your stay patterns.

First, to get Hyatt Diamond you need 25 stays or 50 nights per calendar year I believe, not 1/2 this year & 1/2 next year from when you joined. I'm not totally positive about the calendar year (but that's how most of the hotel programs work), but if I were you I'd check so you don't think you're getting something you won't actually get. Will you have enough stays/nights and/or are there Hyatts everywhere you'll be staying? If there are, then staying w/ Hyatt is the right thing to do. If there aren't, then consider switching to Marriott as your main program due to # of brands & more locations.

While the conferences themselves are being held at Marriotts, are there Hyatts in the near vicinity of where the Marriott conferences are held (there is in SF for example)? If so, stay at the Hyatts & then just go over to the Marriotts for the conference (be it walking, public transport, taxi or rental car).

I've done that before on multiple occasions. Attended a conference that was held at a non-Marriott property but didn't want to give up the Marriott points for a week, so found the nearest Marriott property & stayed there. Wasn't quite as convenient as being able to push the elevator button first thing in the morning & last thing at night at the conference property, but I got my points & that was worth a bit of inconvenience.

So check where your conferences are being held & find out if there are Hyatt properties nearby where you can stay instead & then just head to the Marriott(s) for the conference.

If you have to stay at Marriotts for your conferences but really don't plan on staying at them otherwise, then you have 2 options - signing up for the gold challenge if it will encompass the time frame you'll be staying for at least one of them (presumably the longest) just so you get the bennies for that stay, but continue to book the other stays at Hyatts to continue to go towards Diamond, or just don't bother w/ the gold challenge & book at Marriott as a Marriott member. Marriott points don't expire.

If you're not going to be able to meet the Marriott gold tier qualification post-challenge, then there's no reason to switch your Hyatt stays to Marriott just to meet the gold challenge. You get the gold bennies just by enrolling in the challenge for those 90 days. If you don't hit the stays required, the worst that happens is you drop back to basic status.

Hyatt has a Diamond challenge of 12 nights in 60 days. Have you considered asking them to enroll you in that (if you think you can meet the criteria)? I don't know if it's just for folk from other chains or if they allow their own members to do the challenge.

Whether you do a Marriott or Hyatt challenge, consider the timing of when you sign up for the challenge. The best time to do either is just before your stays/nights will start, in order to extend the challenge for the longest part of the timeframe. For example, if you've got Marriott conferences in August & October, signing up now won't help you w/ the October conference/status, but if you signed up in August it would encompass both. Similar for Hyatt.

The other thing you need to consider when determining chains (and not just the credit card) is what is important to you/what are you trying to accomplish when staying at them. That will help you determine what is the best chain(s) for you, not the credit card.

PS - It's always good to have 2 chains in your portfolio, one as the primary & one as the back-up for the times when you're in locations where one might not be or the rate is too high for the primary.

PSS - If you're attending a conference in SF where the room rate is $600/night, that's a crappy meeting planner. Conferences get reduced rates, not higher rates! Did you just call Marriott to book a room for that night w/o mentioning the conference? If so, call them back & tell them you're attending the conference & to book you at the conference-negotiated rate.

Cheers.