MH normally flies 737-800 regionally. The planes are relatively new and Y seating is on a par with the other OW airlines that I have flown. The longest segment you list is about 1870 miles - so about 4 hours.

As to whether to fly business - Do you need the tier points? Do you have the spare cash? Note that MH does charge slightly less per sector on round trip tickets. And at the moment MH sale fares which do not earn mileage (anywhere) are not that much more than Air Asia.

Happy wandering

Fred