My very first Birkin is 35 in togo and I was very happy with this choice.
It's still absolutely perfect after a few years. The second one was 30 in clemence and I love it best. What surprised me the most was how much lighter it is than my togo. Also, it seemed tiny at first, specially when fully closed, but it tourned out to be so spacious, and I never wear it fully closed anyway...
I think that I am going off the topic here.
I just received the 30 in gold swift, the same as Jasanna has, and it's gorgeous.
The color is very rich and the leather is wonderful to the touch - soft and luscious; but I think that it's much more delicate than togo. I am not sure if I'll keep it yet, since it may not work for me - I have a very active toddler and it's not the right time for me to baby the bag...
Also, if you live in the area with rainy weather, I would definitely go with togo.
Basically, for the first bag that you want to use everybody and take everywhere I would choose Togo (or clemence - my favorite casual leather) over the swift. It also depends on the color you want, with darker colors being more practical in both leathers, but specially in swift.
Good luck to make a right - for you - choice, and enjoy!
Elena