Re: You heard that where?
Um...I don't think you have all your facts right either.
1. Laptimes don't matter - what matters is how difficult it is to jump in and get competitive laptimes with the top drivers in either sport. Not an expert, but from street riding and watching racing, bikes sure seem like a bigger challenge at that level. Plus at ~326 lbs and 240+ hp (Honda), MotoGP currently has a better power-to-weight ratio (1.35 lbs per hp, versus 1.47). Hp ratings are meaningless without looking at weight too.
2. Correction: Rossi spun out in "his first group outing" this year, but has tested well at various other solo outings over the past few years. Plus later in that same outing this year, he posted very competitive lap times (though using a non-current-spec car). I haven't heard how well Alonso's first group outing on a MotoGP bike went yet
The "big mouth" award actually goes to the F1 driver. This is simply Rossi's reply - "you say you're better, then prove it". Seems pretty reasonable to me.
3. F1 drivers are talented, but "most talented"? I'd say that's subjective, and this challenge might be a good way to settle that. Honestly, bikes and cars are so different it's almost like saying F16 fighter pilots are more talented than F1 drivers...
You think Alonso would win, I think Rossi would win. So it'd be nice to see the challenge happen!
That's my take on it anyway...