I've had 4 C-sections and the lap Nissen ( actually mine was a Toupet), and believe me they are not equal! The stomach surgery is way more serious. I had my tonsils out as an older kid, so I remember that too. I wouldn't even consider doing them together.
C-sections are ordinarily done with spinal anesthesia. I was up taking care of babies the next day. Nissen is under general and can take up to 4 hours, depending on the complexity of the case. If you have a hiatal hernia being taken down, that is thoracic surgery on top of the abdominal compnent. You have all sorts of stuff shoved down your throat - the endoscope, ET tube for ventilation, a bougie for sizing the wrap, and you end up with an NG tube for the next day. You won't feel like getting up the next day.
You'll have general anesthesia for the tonsils too. You can swallow a lot of blood and be gagging and throwing up afterwards, although an NG tube could possibly help with that. This makes me think that it might be a good idea to get tonsils done first. You really don't want to throw up after your Nissen!! It could mess up the whole wrap and make them have to go in and do it over.
I'd trust the opinion of the surgeon doing the Nissen.