What is the difference between the following two phrases?
Be’ wyt ti’n gwneud penwythnos ‘ma?
Be’ wnei di penwythnos hon?
Do they both mean, “What are you going to do this weekend?” And, what is the difference between using 'ma and hon?
Be’ wyt ti’n gwneud …? = What are you doing…?
Be’ wnei di? = Short form for - What will you do …?
(y)ma and hon both mean this.
So the general meaning is similar, although put slightly differently.
You could also say - What are you going to do?: Be’ ti’n mynd i 'wneud?
'ma (short for ‘yma’ - literally ‘here’) is more colloquial than ‘hwn’ (masculine) or ‘hon’ (feminine).
I would say ‘y penwythnos hwn’ rather than ‘y penwythnos hon’ but for some Welsh speakers ‘penwythnos’ is feminine (hence requires ‘hon’) rather than masculine (‘hwn’).
But either way, the meaning is ‘this weekend’.