Well, the British have access to the Sherman via Lend Lease and they designated it the M4A4 or Sherman V but I believe the chassis is basically the M4A3
IIRC. More importantly they put a 17 pound gun on it and called it a Firefly. It basically made the Sherman a tank destroyer.
http://en.wikipedia.org/wiki/Lend-Lease_Sherman_tanks
The British tanks themselves tend to have a really bulky look to them. Their tanks are solid and most benefit from the rule semi-indirect fire making them good defensive pieces when needed.
Americans are essentially variations on a theme. The Sherman is nothing spectacular from it's base M4. When you start looking at it's variants, the 76mm, the 105mm, M4A3E8 "Easy Eight" and the M4A3E2 "Jumbo" things start getting interesting but expensive point wise. The 76mm brings up the punch of the Sherman significantly to where it can actually hurt Tigers. The 105mm won't do well against tanks unless you can flank em, but will clear out infantry as it's a break through gun. The Jumbo has very good armor and a 76mm cannon making it very good when combined with the Jumbos Lead the Way! special rule. It essentially makes them draw fire. The Easy Eight has a plethora of nice things going for it from a 76mm, gun stabilizer, movement rules and slightly better armor.
Americans also have nice light tank options in the M5A1 Stuart and the Chaffee.
Americans have the M18 Hellcat and the M36 Jackson Tank destroyers which don't have to be on the field until you want them to be via the American TD rules. It can cause an unaware or distracted opponent to suffer heavy tank loses if theyfail to deal with them properly.