A CBR will be more involving to ride than an underbone. The riding position allows you to "tuck in" and attain higher speeds, and you can carve better in corners, as the pegs are up higher and your weight is more centered, giving you more control of weight balance in the corners. Then again, an underbone is more relaxing to drive in traffic, as you aren't hunched over. Your choice.

A CBR 150 or 250 is an excellent beginner's bike if you're eventually planning to go for a big bike... but seeing as how there are very few places you can actually use the extra power of the big bike, and the fact that a CBR 150 alone is much quicker at traffic speeds than most of the cars on the road, you might not even feel the need to trade up... not unless you're planning to take long highway trips.