Kagoshima to Tokyo on JR Pass

Hello! I'll be traveling from Kagoshima to Tokyo on a Saturday (13th April) and I'm trying to figure out if it's worth upgrading to the Nozomi/Mizuho shinkansen, or if the JR-included shinkansen are fine. Is there much difference in the speed? Are the Nozomi/Mizuho more fun(!?) Or will it be much the same trip (but a little longer) on the regular semi-fast shinkansen? Does anyone know the cost/time difference? I cannot figure out if you pay a surcharge on top of the JR pass, or if you have to buy a whole new ticket to catch the Nozomi/Mizuho. I'm a little bamboozled by all of the timetable connections on Hyperdia, so thought I'd ask a human being, thanks in advance!

I'm trying to figure out if it's worth upgrading to the Nozomi/Mizuho shinkansen

You cannot upgrade. The Nozomi and Mizuho are not covered by the pass, at all.

or if the JR-included shinkansen are fine.

Yes, they are fine. If you take a Sakura and Hikari via Shin-Osaka, the travel time is less than an hour longer than when taking a Nozomi.

Is there much difference in the speed?

Hikari trains stop a few times more often than Nozomi trains. Sakura trains are almost as fast as Nozomi. Mizuho trains are very infrequent and somewhat faster than Sakura trains.

Are the Nozomi/Mizuho more fun(!?)

No. But Nozomi trains generally have the newer train sets than Hikari trains.

Does anyone know the cost/time difference?

http://www.hyperdia.com/

More basics:
http://www.japan-guide.com/e/e2361.html

Arigato Gozaimasu!
That answers all of my questions, Uji.