I have two tractors as well, a 50HP and 20HP. Power is only one aspect of the “work” equation. Weight is the other. It wouldn’t do any good to double or triple the HP on my small tractor. It’s weight (i.e., traction, strength, stability) isn’t sufficient to do significantly more work.
I agree that it makes sense to get the largest, heaviest, most powerful unit that you are comfortable negotiating between your full grown rows (and that you can afford). The subcompacts (i.e., > 25HP) won’t pull a disc heavy enough to work effectively (even a 48" wide one), or accommodate a decent 3pt blast sprayer. On harvest day, all the small tractor will be good for is towing a cart with a couple of 1/2 ton bins. That said, I love using the small tractor for the things it can do because it operates so easily within a 6’ wide interrow.
Speaking of harvest, I am very glad that the loader on my larger tractor is robust enough to handle 1/2 ton bins. I’m solely a grower though, so moving barrels around has never been a consideration.
If you are evaluating loaders, understand that ratings usually apply to pivot points, not to the ends of pallet forks. If you can, test out the loader you intend to buy on some fully loaded 1/2 ton bins to make sure it can handle them comfortably.
Also, if you elect to get a loader, a “skid steer” type attachment interface is priceless. It will allow you to instantly connect and disconnect an infinite array of standard loader attachments, include a bucket or pallet forks. It’s is highly likely that by simply removing the bucket (or other attachment) from your loader before you do work between the vine rows, you won’t find is necessary to remove the loader itself (which is a hassle of sorts, no matter what). The skid steer type interface (I can’t remember the common name for it) will allow you to remove whatever attachment you have on the loader quickly, so you can get to your row work right away.
BTW, I love R4 tires for vineyard work (if they don’t increase the width of tractor beyond the maximum you can tolerate).
Finally, I agree with the comment that there is no substitute for an on-site demo of anything you’re thinking of buying. If it’s a larger tractor you’re considering, take it up and down the rows a bunch to make sure you’re comfortable with the size.