I think your experience lines up with exactly what you should expect. Compared to the Street, the Sportster is indeed heavy, slow, and the vibrations are far more prevalent. That's really the appeal of the Street -- it's a Harley that's light, nimble, and agile. Generally, Harleys aren't known for their agility; the Street fills that niche by being lithe and tossable, making it suitable for urban commutes not just in America but in countries around the world like India, where the crowded streets are so much more crowded you'd hardly believe it.
As for the Street Rod -- I'd like to have its engine, but other than that I prefer the Street 750. The geometry, riding position, low handlebars, all that stuff may make the Street Rod a better "racer" when cafe racing, but I much prefer the riding position on the Street 750 -- it's un-cramped, relaxed, and unlike just about every other Harley, it doesn't require your feet to be splayed way in front of you. The Street Rod has had its riding position mentioned in a few reviews, usually not very complimentarily; I saw a Street 750 with drag bars and I thought it looked phenomenal, until I tried to sit on it and I knew that my middle-aged back would despise the riding position after only a few miles. To be honest, the riding position on the Street 750 is one of the things that got me back into motorcycling; I can't stand the "fetal position" you need to cram into for a modern supersport, and I always wanted a Harley but couldn't stand the feet-forward riding position. The Street 750 fits me just fine.
There's also height considerations; the Street Rod's seat is a lot higher than the Street 750's. Shorter folks may not fit on the Street Rod at all, but just about anyone can fit a Street.
Don't get me wrong, the Street 750's far from perfect; I've spent quite a bit changing it out and fixing it up with better mirrors, seat, grips, performance upgrades, etc. In stock form, I thought about trading it in after a couple thousand miles, but I'd take a loss one way or the other so I thought I'd spend that dough on upgrades and I'm really rather happy with it right now as it is.
And I'm sure there are just as many happy Street Rod owners who are thinking "no way would I trade a Street Rod for a Street 750." Which is why we should be happy that both models are on the market, so both customer bases can get what they want.
(and if you do decide to upgrade: as a suggestion, instead of a Sportster, consider looking at the Low Rider S
; it's like a super-ultra-mega Street and has some of the best reviews I've ever seen any Harley get).