Clips won't affect how well the straps hold you in -- all they really do is position the toe strap so it's convenient to flip in and out (on the track many riders use toe straps without clips because they don't have to worry about convenient pedal entry at every stoplight).

As pointed out above, if you don't have cleats, don't expect straps to lock you in.

The only other thing is that you can set up straps in different ways, some of which leave a lot of elasticity in the straps. If you twist the strap inside the pedal cage, or have it run in and out the back of the rear cage plate, then it won't be as tight as when it just runs a straight line around your foot.