Seishun 18 or region pass

I'm coming to Japan in July for 2 weeks and need advice on which pass I should use.

My draft itinerary is:
1 week in Sapporo and 1 week in Tokyo. I'll be spending most of my time around the city (I'm planning to use my sucia card).
I'm planning to have day trips outside the city.
While in Sapporo, I want train to Kushiro and Asahikawa as a serpate one day side trip.
While in Tokyo. I want to train to Tateyama in Chiba.

I was thinking I could buy JR Seishun 18 Kippu and use it for when I'm in Hokkaido and Tokyo. But I realised you can't book reserve seats.

If you've visited either of these areas, my questions is. Should I buy a regional pass to get the reserve seats, or save money and buy the Seishun 18 Kuppu?

Thank you in advance!

18 is not good for Hokkaido travel wide.
BC simple reason slower trains(local and rapid) operate not a lot.
http://www.hyperdia.com/en/
Basically 18 not valid for LtdExp/Exp trains and High(Exp)way buses.

18(2015's) details info update yet.

18 for in Tokyo and to Tateyama not so bad but a little too much maybe.
In Tokyo(area wide) placed a lot of non-JR railway lines(subway,Private rail and more) exist.
http://en.wikipedia.org/wiki/Transport_in_Greater_Tokyo
http://www.skyscrapercity.com/showthread.php?t=828904&page=325
18 also not valid for non-JR railways and local buses.