The 1250 will work great with a 44 inch deck. The 1650 will only allow you to clog it up quicker with clippings.
I use a 48 inch deck with a 12hp 108 (didn't re-badge it as a 128) and 50 inch decks on both a 1450 and a 1650. Honestly, they all cut about the same.
As for tilling (and mowing, too, I suppose) your use of power is constrained by the single PTO belt. And tilling, application of more HP to the tiller tines will only throw the tractor around more. Same situation with my Kubota back hoe. If it had more than 23hp, it wouldn't help as the application of more power to the soil simply picks the machine up off the ground. It isn't heavy enough to take advantage of the extra power. Heck, it's not heavy enough for the 23hp it has.
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