The Contra boys are great for exploring the SMBS levels too!
Now something that should be mentioned is that they added an assload of options to customize your play through. One of the best for me is the ability to adjust the amount of damage the characters do. I always thought it was a bit wierd that a goomba takes multiple bullets to kill so now when I play with Bill I just turn the damage up. Another thing I liked is that you can choose the number of starting lives. I like turning it down to one to make collecting coins a smidge more important.
Speaking of coins it seems like the SMBS levels have less coins compared to SMB1, but the bonus rooms tend to have loads of coins which is a good balance, I think.