Would the avast boot time scan not enought to deal with the rootkits?
nope....
the boot time scan you use if the normal quick/full scan detect a malware and have difficult removing it
Avast Antivirus offers a "boot time" virus scan of your PC. This allows the antivirus engine to scan all of the files on your hard drive before any other programs load - useful in cases where you have an infection which cannot be cleaned because the "file is in use"
There is no contest as they are two different things.
The aswMBR is a specialist tool looking for one thing MBR rootkits. Then production of a log, which requires analysis by someone knowledgeable in its use as to how to proceed. So this is going to need more than one pass/run to detect and clear the infected MBR and even then there are occasions where other tools are required if what is found can’t be resolved by it.