If I have a form in which most of the controls are bound to form properties, and I want to conditionally change the value of one property based on the user changing another should I change the control's value or the form property?

eg. Say I have a check box, chk1 bound to thisform.lChk, and a textbox, txt1 bound to thisform.cText
If the user types anything into txt1 I want to set the check box on. So... do I say
thisform.lChk = .T.
or
thisform.chk1.value = .T.
????

And for a bonus, why?
TIA