Good morning, Ruth. I grow both of the ones you mentioned, plus vitafolia. They all bloom well for me, but they die to the ground and are late coming back in spring.

Lady margaret blooms the best and most for me - from June til our first freeze, but the blooms are much smaller that the other two. Lady margaret is a manageable size vine, while the other two grow into massive vines.

ruthz,
Not sure if you've bought one yet but there are some other things to consider. If you don't want caterpillars to consume your plant you are better off with the crimson, aka grape leaf passion vine as seen in the above photo. It's real name is p.vitifolia. It produces nice "tiny" watermelon colors type fruit when hand pollinated.
P.Lady Margaret is a great plant as well and is a host plant for gulf fritillary. The caterpillars will eat this plant up quickly and then you can admire the butterflies. This one does not often produce fruit.
That said, I give a slight edge to passiflora vitifolia because the large luxurious red blooms.