Why you want MNP ? Both Digi and Celcom now using same network in Klang valley in 70% of places. Either you subscribe Digi or use Celcom you get same experience.
B4 port in Maxis just buy sim card on 99 try the coverage

Then go for it. CD Prepaid 25 definitely value for money. I think most places Digi improved in terms on indoor coverage mainly

From my experience having used Maxis previously before switching to CD, outdoor coverage for 4G more or less the same. However, for indoors I find CD have better speeds compared to Maxis.

I mostly around BU, and indoor although both have good 4G coverage, CD is faster. Maxis I noticed within this area is that got full 4G bars, but Internet very slow. Got frequent buffer while watching FB videos/Twitter using Maxis.