Depends on your area, you may try getting a prepaid SIM pack to test or ask anyone around from your area that's using U Mobile. Personally I feel the network has been stable in KL most of the times, although there are some hiccups here and there especially in MRT underground tunnel or some basement car park.

As for Celcom, are they forcing you to upgrade to their more expensive plans? If that's so, this is not their first time doing this, you have to threaten them that you'll be reporting to MCMC for unfair business practice because they have no right to stop you from upgrading to any plans regardless of how old your current plan is.
The only difference is that Postpaid 38 is full speed but with 30GB data while Prepaid U35 having speed cap at 6 Mbps but with "unlimited" data (100GB FUP).

Ah yes, still using the same exact tactic to trick loyal users, it feels like they're trying to "penalize" loyal users for sticking with old plans for so long so they have force loyal users to jump straight to their premium plans, very dirty business tactic right there.

But anyhow, hope you're satisfied with U Mobile.