I have the Ace 250 with the stock coil, the 9"x12" coil and the 4.5" Sniper coil. I used them all under different circumstances. I use the 9"x12" coil in the dry sand at the beach and in large, relatively clean areas. You can cover allot of ground quickly with the 9"x12". The 4.5" sniper is great for trashy areas. Thinking about it, I rarely use the stock coil anymore.
Back to your original question, yes, I think the 9"x12" coil is a must-have. I'd say the same for the 4.5" Sniper coil too, but that's not your question.
Is it worth the difference in price? You'll have to decide that. The coils usually run around $85 new, so look at both deals and see if you'll be saving money or not. If not, you can always get the 9"x12" at a later time. Also, perhaps there are other valuables that come with the higher priced deal.
Edit: I just realized that I forgot to add the point that you do get some additional depth with the 9"x12" coil over the stock coil, so if you are looking for deep items, the 9"x12" will help.