You would be fine with either for Blues music. It really comes down to the tone you want. If you want a bit of a dirtier, raunchier tone (along the lines of an Albert King). Then I would say the P90's would be closer to what you would want. I also know that Jonny Lang plays a Tele with P90's. If you want something that would be a little on the cleaner and clearer side (a la Buddy Guy, SRV and such) then go for the strat.
That being said, if you have a chance to play both types of guitars, that would be the best bet. Try them out and see how easy it is to get the tone you are looking for and which one you enjoy the most.
I honestly have played a strat and guitars with P90's and like both. Currently have neither but plan on getting both styles in the future. A strat is just a classic style guitar that will fit with any style of music. Then you can't beat the raunchiness of a P90 guitar. Especially a Les Paul Junior with that one P90!! That being said, you can get an excellent clean out of a P90 as well.
Not sure how much help I was, but good luck in your search!!