Is an Emax crown on tooth 7 worth the additional $250 compared to a metal one covered in porcelain?

You are getting a steal at that cost for an Emax crown!  It is a much better choice than porcelain to metal.  It is both extremely aesthetic and durable.  Don't think twice.  Get it done.  Good luck.  

E.Max is one of the strongest
ceramics we have available in dentistry and certainly more aesthetic and
natural looking than a metal supported porcelain restoration. The e.Max
proclaim is 4 x stronger and less resistant to chipping and fracture than the
porcelain placed on the metal supported porcelain crown. The first question you
should ask your dentist is why now a crown and not replace the veneer with
another veneer? If the porcelain chipped on your 20 year old veneer, it
would be best just to remove the rest of that veneer and prepare your tooth for
another veneer and I would choose e.Max for this. The e.Max can be bonded
to your tooth and thus is more retentive and also not as much of your natural
tooth structure will have to be removed compared to the metal supported
porcelain

The cost for this is not out of line.  The value of it depends on how well it matches the teeth around it.  Emax, or porcelain, has much more lifelike characteristics than a crown with metal in it.  Light does not diffuse through metal and thus it can make the tooth look unrealistic.  I would choose to place an emax on this tooth.

I rarely do a porcelain fused to metal crown on an anterior tooth any more.  Use to be this was all we had but the esthetics were not as good as the newer materials like Emax.  Emax is now my material of choice for front teeth. And the price they are charging is quite reasonable.  I would suggest going for the Emax.  Good luck!