I'm on 38 plan. Celcom 3G and H+ data are very unstable since the introduction of first 38 plan. you need to have at least signal strength of - 90dbm or better to have smooth Internet. Celcom 4G works great for me. Smooth Internet with only 1 bar signal or about - 115dbm. Guess anything lower than 4g is just too congested for most location atm.

frankly speaking I don't find the switching between 3g and 4g is slow. 4g switch to 3g instantly whenever I make calls with no delay. however, switching between 3g and h+ seems slow if signal is not very good.

Agree. It does matter. 2 different devices reporting the same signal strength does not necessary provide the same performance and stability.