.30-06 Springfield or 300 Whelen which is better?

For sure the 30-06 and the 35 Whelen necked down to 30 caliber are the same - or seem to be. Typically though, when confronted with a simple answer, one should always be expected to consider it wrong - because it almost always is.
So decades ago we had great success with the 7-08 in production rifle silhouette because we could get good .308 brass which when necked-downed and properly neck reamed would get a better chamber fit with bullet alignment. Yep we tried the same thing with getting good .358 brass and resizing to .308. The issue is with getting good .358 brass, but if a person has enough money, time, equipment and enthusiasm, things can be made to work well. That is a huge contrast to the 7 TC/U where one could form some .223 brass and fire-form while shooting 200M chickens consistently with a handgun. Of course there would be differences in POI between brass ( we all used the Hornaday 150 IHMSA bullets which were great!) but we sighted for that and just had to discard the brass of misses for analysis - all brass being match prepped. We had great fun with a friend's .303 British in a gun rebuilt by one of the big British gun houses for use abroad and so made to proper specifications with regard to bore and chamber in a new barrel. That gun with proper reloads sure opened some eyes in open sight competition...

What I'm trying to get at in my old fashioned way is that yes I probably could get your old gramps 30-06 to shoot better with some proper hand-loads, with properly set up 35 Whelen brass neck sized. That being said, I've extensively shot an old P17 gramps gun of which only the action was kept. it got a new stock (hand bedded) and a new high grade barrel in 338-06. And it got lots of action and trigger work. Well with only the action remaining and using a cartridge necked up, and using an ancient P17 action with terrible lock time how good could that be? It's incredible even for someone used to a Jewel trigger and fast lock times! So now we have gramps gun re-worked. For my friend it was his gramps gun. He changed it to work for him and then I took him to go get a moose with it Now my friend has his own grand-kids and so things continue.