That is a very general question, and I will give my opinion in a general format. I would like to address your last situation first, the 'net deck'. I believe that copying someones successful spell book is not as detrimental to Mage Wars as some other constructed card games, like MTG, hearthstone, or Netrunner. The reason I hold this to be true is that Mage Wars, like most good games, is a game of decisions. Each turn the player needs to choose the best spell from the spell available.
The 'net deck' gives them the tools, however the proper use of those tools are dependent on the player not the spell book. So a well stocked tool box will not help the player if he uses an wrench as a hammer.
Now on to you first question, limited, but well known book or a broader book. I am making an assumption that the familiarity with the spells are similar and the difference between the two is one of the pool of available spells. The limited book has very specific spells and tactics for winning the game, a well versed opening, a solid plan for the mid-game and a method for winning. Where the broader book is more flexible in its opening with several choices available, more tools for responding to the opponents opening and open-ended on finishing the game.
With in the parameters listed above, I believe both will be good in different situations. Verses the unknown opponent, for example a tournament situation, the limited book may be better. It forces the opponent to respond to your plan. However, in a situation where you are playing against a smaller pool of opponents or a wide variety of mages, where you spell book and associated tactics are known to the group a broader more flexible spell book would be of more use and allow you to adapt better to an unanticipated situation.
But in any case the most important thing is to have an enjoyable game, one that you and your opponent will find memorable. Winning at the cost of enjoyment is not worthwhile, in my humble opinion.
