Butyl Hydroxy Toluene (BHT) has two tert-butyl groups that can stabilize a free radical. Butyl Hydroxy Anisol (BHA) doesn't, though it has the 4-substituted methoxy group (I'm not sure what that does).
What is the stronger antioxidant?
That's a good question. In literature there are some contrasting opinions about the antioxidant power of BHA vs BHT. TBHQ (2-tert-butyl hydroquinone) is also used as antioxidant, although maybe is less common. There is a paper from 1997 in which the antioxidant activity of BHA, BHT and TBHQ are examined with Miller's test (M. Karamac and R. Amarowicz, Grasas y Aceites, 1997, 48 (2), 83-86), and in this test BHA and BHT gave similar results, although in the initial phase of the experiment BHA seemed to have a higher antioxidant activity than BHT. In the paper precedent studies are also reported. Quoting from the paper:
Higher antioxidative activity of BHA than of BHT and lower for TBHQ was demonstrated in thin layer model system of methyl linoleate on cellulose (J. Am. Oil Chem. Soc. 52,174-178). Berner et al. (J. Am. Oil Chem. Soc. 51, 292-296) reported antioxidative index in lard emulsion 30-fold higher for BHA than for TBHQ. Yet, in active oxygen method, antioxidant index for TBHQ was 3-fold higher than for BHA and BHT. Higher antioxidative activity of TBHQ compared with BHA and BHT in model studies with soybean oil and chicken fat in emulsion versus dry oil tests was reported by Cort et al. (J. Am. Oil Chem. Soc. 52,174-178). Relative effectiveness of antioxidants-linoleic acid monolayer on silica gel for TBHQ was almost the same as for BHT, while for BHA almost 3-fold higher (J. Food Sci. 42, 1533-1535).