Tome one contains more of your "Utility" spells, such as; Dissolve,block,dispel,ect.
Tome two contains more of your "Powerful" spells, such as; Major equipments, spawnpoints, high cost creatures.
If you just want it for you to build decks, tome two may be the way to go.
If you are looking for spells for more then just yourself, then one is a good bet.
Here is a link to a list that shows what comes in each one, as well as showing you what you get if you own two core sets in comparison to the tomes. (Which, if you can get two core sets over the tomes, that is the way to go for sure.)
http://boardgamegeek.com/filepage/86416/basex2-vs-basecore-tomes