It is lovely to have a box to open on Christmas Day, but...what if the contents aren't quite what you had in mind?
Does your wife know about this proposed gift? Has she had any input into the requirements?
Less exciting on the day, but perhaps worth thinking about is a gift wrapped 'voucher', promising to take her to a real sewing machine shop to look at, try out and decide just what she wants (subject to budget of course). As others have said, a sewing machine is a personal thing - it has to work the way you understand, not insist it knows best; it has to have the features that are important first, any other bells and whistles are purely incidental. The other advantage of a real live shop (not necessarily a department store, though that might fit the bill) is that you can talk to someone who knows the products and should be able to show you the various features and explain anything you need to know. It may also happen that your wife might after all be more amenable to a pre-owned machine that does exactly what she wants rather than a new one that isn't quite right. Second hand machines usually come with some sort of warranty, and establishing a relationship with a shop that can provide advice, servicing and extras is a good thing.