I see some advantages to having "twin" street and track bikes, but personally, I kinda like having two different bikes... It's fun going back and forth and experiencing the similarities and differences.
Both are v-twins, but...
My street bike is upright, race bike is leaned over.
My street bike is 998cc's, race bike is 650
My street bike has 115rwhp, race bike has 70
My street bike is regular shift, race bike is GP
It also kinda helps me remember where I am and reminds me to ride accordingly.
As far as popularity goes, pretty much any bike manufactured by the big four will have plenty of parts and support available to you, so I really wouldn't get too caught up on that.
And don't discount the ole Sv650! I'm a bit biased, but I find it easy to ride quick but challenging (in a good way) to ride fast. Definitely a great learning tool and inexpensive, which leaves more money for track time
I say just keep your budget in mind and buy what you like!