I've always been impressed with the iPad's standby battery life. Of course, it's doing pretty much nothing while sleeping, but it's admittedly very nice to pick up my iPad Air 2 when I haven't used it for a couple of days and seeing it at 99% charge.
That said, of course an iPad of reasonable age will get much better life than an SP3. I should hope so, given the differences. 12.5" screen, Intel Core processor, desktop-class RAM and SSD, etc., etc. Everything about the iPad is engineered to sip power, which is not and should not be the case with the SP3. I'd much rather have a machine that offers decent battery life and full power when I need it than one that's been crippled just to last as long as possible on a charge.
I like my iPad, and use it for very specific reasons (such as, because I just don't feel confident carrying my SP3 around with me quite as much, being a $1500 investment). But longer battery life isn't something I consider to be such a huge advantage given how much more my SP3 is capable of doing.