Both should be fine. Taw is a southern dialect word. You could also use "mai" instead which is the northern form.
mai/taw, but not bod...
mai/taw is used to link an emphatic clause such as Megan yw'r dala with the preceding dw i'n meddwl - linking using maw/taw allows the ...yw... to be used. If we use bod to do the linking, there cannot be a verb in the Megan... clause:
Dw i'n meddwl bod Megan yn dal - ' - I think that Megan is tall' - no verb in ...Megan yn dal
Dw i'n meddwl mai/taw Megan ydy'r dala - the verb ydy can be used to keep the emphatic structure with Megan..
Dw i'n meddwl fy mod i'n araf - I think that I'm slow - normal, unemphatic second part of the sentence after ...bod
- Dw i'n meddwl mai ^ fi sy'n araf ^ - I think that it's me who is slow. (Emphatic pattern in the second half of the sentence between the ^ marks, so mai/taw must be used to link to it, allowing the verb sy to remain as part of the emphatic pattern.)
(As EllisV says, mai is the 'standard' form here, taw is also used in parts of southern Wales. Taw is quite handy to learn and use, though, as it makes it less likely to get confused with mae.)