Visited both yesterday.
Haus der Musik (The House of Music) is a very entertaining museum on creating and combining new music from different kinds of sounds with electronic tools, as well as an introduction to Austrian innovation in musik from Heydn to Schoenberg and Berg etc. Good for adults and children, a definite must even for a sunny day.
Prater is still the same old-fashioned amusement park including two of the few surviving tall, slowly spinning statues of exotic types originally promoting smoking I first saw in Disney’s Pinocchio movie. So we really visited it as in "looking around at both displays and people". An entertaining learning experience.
The main noteworthy feature for fast ride lovers is there are a couple of rides for very few people combining great views (if you can manage noticing them while bracing for the next acceleration) with very fast speeds and significant accelerations. Also accelerations are always back and forth with no side pushes, which is great for middle-aged riders who still need time to grow up to their new physique.
I tried "Turbo-booster" the one where two 4-people gondolas are at the ends of a 10-meter (my guess) arm which is turned one way for a minute and another way for another couple. Gondolas are on a horizontal hinge, so you get your gondola to spin, with or against the main spin. 100 km/h reported, wind in you hair does feel stronger than your run of the mill ride. The other interesting one is "Ejection Seat" – look it up. Reports throwing you at 90 meter height in a 2-person cage on 2 strings.
Please note all rides are paid individually, these are among the most expensive.