a lot may depend on where you will be shooting... outdoors or indoors... the 70-300 may not work indoors with very little room.... and it may not be fast enough for indoors...
you might want to check into the canon 18-200, sigma and tamron also offer it... great range.... if you don't need 2.8 or better for low light situations...
my fav lens is my canon 70-200 2.8 IS.. but I can't use it in my house for studio type shooting because I don't have the room... so I use my canon 28-135 .. if you look in photo sharing you can see some of mine with these lenses...
good luck!
"I expect to pass through the world but once. Any good therefore that I can do, or any kindness I can show to any creature, let me do it now. Let me not defer it, for I shall not pass this way again." Shephen Grellet 1773-1855