Huge different bro:-

Power Prepaid + UMI30
Voice = 10cents per minute
Data = 2.5GB
Free voice all network = 30 minutes
Free voice to uMobile = 450 minutes
No billshock
Total = RM30

Power Prepaid only
Voice = 10cents per minute
Data = 1GB
Free voice all network = 300minutes (from RM30 credit)
Free voice to uMobile = 450 minutes
No billshock
Total = RM30

U28 Postpaid
Voice = 20cents per minute 
Data = 3GB
Free voice = 50minutes
Free voice to uMobile = 0 minutes 
Billshock is possible
Cannot carry forward free minutes 
Total = RM30

Conclusion
U28 is a con plan after Power Prepaid is out.