I've never been to that museum. But from the Cadorna station to the Fiera/Rho with the Metro 1 Rossa all you need is 15-20 min. Same with the Passante train from Porta Garibaldi to the Fiera/Rho train station. From there I don't know how long that bus would take, but probably 15 min, with a taxi is maybe a little less.
If you take a taxi all the way from downtown Milan, let's say Cadorna station, it might take you 25 min altogether, is there is no traffic.
If you do not intend to take the bus linked above, then I'd take the taxi from downtown. It's not going to be very cheap, maybe 35 euro, but it will be the most convenient option.
Uber doesn't really exist in Italy, or not the way it works in the US.
The taxi lobby managed to make Uber illegal in Italy. The only Uber service available is Uber Black app. Uber Black is a luxury black limo service which will be fancy and nice, but always more expensive than a regular taxi. They are basically black Mercedes Benz driven by a licensed professional limo driver in a suit. The reason why Uber operates only that service is because a court in Italy established that that type of service can only be provided by professional licensed drivers (like taxis or limo service), not by the average Joe Blow or Jane Doe using his/her beat up car. It was a victory for the taxi lobby when that supreme court decision came out in 2015. So unless you are in need to travel in style, you'll probably save money by using a regular taxi over Uber Black.