Since there have been changes, which is better now - the Museumkaart or I Amsterdam CIty Card

Well, the only one who can really answer that, would be you. Take a look at the regular entrance prices of the attractions you would like to visit and compare that to the different cards.

Most public transport in NL is now cashless, but you can still buy single tickets on the tram using your credit card.

the basics:
Museumcard: 60 euros for 5 museum visits. Valid on almost all museums in NL, public transport not included

I Amsterdam City Card: 59 euros (24h), 74 euros (48h), 87 euros (72h), 98 euros (96h). This card includes GVB operated transport. See for list of free entrance: https://www.iamsterdam.com/en/i-am/i-amsterdam-city-card/what-is-included/locations

GVB pass: 3 euros (1 hour), 7.50 euros (24h), 12.50 euros (48h), 17.50 euros (72h), 22.50 euros (96h), of course only valid on GVB operated transport. Not valid for any attractions.

If you plan to travel outside of Amsterdam (eg Schiphol, Zaandam, Haarlem), the Amsterdam and Region Travel Ticket is also an option. This ticket includes transport by train and transport operated by GVB, Connexxion and EBS. Not valid for any attractions.. https://www.iamsterdam.com/en/plan-your-trip/getting-around/public-transport/region-day-ticket and https://www.iamsterdam.com/media/pdf/visiting/artt-map-2018.pdf?la=en
18.50 euros (1 calender day), 26.00 euros (2 days), 33.50 euros (3 days)

It depends what you want to do in Amsterdam. I’m going soon and added up admission prices v the cost of the iAmsterdam card and the card was worth it for me to buy, so I did.

Thanks Everyone!

I loved the mobility and ease of using the Museumkaart - I didn't have to spend all day in one museum. I could go in and out - Rijksmuseum comes to mind, since my lodging was just down the street from it. And wherever you are in A'dam there's always a museum near by with a clean restroom! :) Based on the small differences in cost vs. card and I do want to go as many times as I want - I think I'll get the I Amsterdam City Card. If I buy online before going it looks like I get a discount. We'll see how it goes for this trip!

Similar to the OP, I did the math and the I Amsterdam Card makes sense now vs. the Museumkaart when I add up the cost of the museums we plan on visiting. There are a lot more than 5 museums on our list (we are in AMS 7 nights) and that's before we add in the transit benefit. So with no TripKey and the Museumkaart limited to 5 museums it's worth figuring it all out. Glad I took the time to do the math as my planning for our September visit had all been done before the changes to the Museumkaart and the disappearance of TripKey. I thought I knew what was the best value. Not so much anymore.

@KAS Just a minor point about Tripkey. It still exists, but it is incredibly inconvenient now for most tourists. You can't reserve a card in advance and the only place you can pick it up in Amsterdam is at a cruise boat ticket office in the center of the city--not at the airport or the train station. Until they get their act together, if you're staying in the Netherlands for a while and you want to use trains as well as buses and trams, you may want to consider an OV-chipkaart.

@T: Thanks for the clarification. We've already purchased our IAMsterdam card which turns out to be a significant saving on museum admission - €45 for each of us based on the museums we plan to visit, not to mention the tram/bus rides. We are dedicated walkers so we might not end up on the tram/bus that much. We are taking the train to Leiden and to Rotterdam but will just purchase tickets at the station. I'm sure glad I took the time to do the math. €90 is a lot to save.