Here's why I think you shouldn't spend (much) more on your own son/daughter than on his/her spouse.
To you, of course, no son- or daughter-in-law comes close to your kids in importance. Your kids are the sun and the moon and the stars. Which is great and as it should be.
But to your adult son/daughter, the person sitting next to him/her on the couch, who they married (or settled down with) -- that person is the sun and the moon and the stars.
So giving in equal amounts to both of them is how you acknowledge the importance of their marriage, how much you value it, how much respect you have for it, how glad you are that they found each other. They have entered into a deeply committed relationship of mutual sharing and support, through thick and thin. And it is your responsibility as a parent and parent-in-law to support the health of their marriage. Contrasts like this "Look, I got this awesome present from my mom, because she wants me to have nice things in my life. You did not get an awesome present from her, because it's not important to her whether you have nice things in your life. Even though my life and your life are completely intertwined." do not support or respect the health of their marriage.
I also think that if you give much more to your adult child than to his/her spouse, you're doing something that might well come across as a little bit territorial or a little bit possessive. I don't mean you are necessarily trying to say to the spouse "he/she was mine before he/she was yours" "you're just this random person he married" "you interloper" etc. But by giving to your own child on a much bigger scale than you give to the spouse, you're making a point of privileging the relationship you have with your adult child over the relationship he/she has with his/her spouse. And I do think that you should not make this point.
Again, of course you will love your own children more! But I don't think you need to show it through uneven spending on presents. They know. What they need is to know that you also love and welcome their spouses. They need to know that when you're all together, this is family time for everyone, not "Time with X's family, and he brought the wife," so everyone can relax and be happy and there's no need for X to worry about whether Mom's feelings are hurt b/c Wife is taking up too much of his attention or Wife's feelings are hurt b/c Mom isn't really completely including her. I would bet (putting aside cases where X, Wife, or Mom is an unreasonable asshole) that this is the best and maybe the only way to stay really close to your kids once they marry.
Disclaimer: I don't have children, let alone adult ones. This is just what I think, based on my own parents & ILs and what I hear from my friends about theirs. And sorry for length 