Both are rated as the easiest English Vocaloids to use so overall it really doesn't matter which you get. Aside from the obivous "one is male...", Big Al has more notes than Ann and is smoother, he also has Br1-Br5 (you'll never use his breaths).
Ann however,despite being more choppier than he is, is relatively easier to get a decent sounding song out of her. Her voice is generally taken as the standard for her era and you can get a decent results out of her without as much adjustments, which has always been her strength over Prima and Sonika.
In the end, both has their merits and charms. I'd recommend Ann, Big Al tends to be a better "second" Vocaloid then a primary. When he has another Vocaloid to lean on, it helps him. Do note; both have accents and you'll have to learn to use them. Though Ann's accent serves her well when you need a female vocal to sound more fuller and mature.