Depends on your scale preference and what you want out of your collection. But for me, I've a fondness for the Alternity guys.... Sleek lads, sleek
I felt like the larger Alternators line was capable of better quality, even if some of the designs didn’t capitalize on it. Alternity had some cool designs that weren’t executed very well, and I wonder if the smaller size contributed to that. I’d still like to pick up an Alternity Cliff, if for nothing other than the glorious head sculpt.
Only Alternity figures I have are Starscream and Gold Bug, since Hasbro never sold them they're harder to come by, and they've never been a big priority for me since the variety is so limited (even owning only two figures I already own two-thirds of the molds made for the entire line). So I gravitate toward Alternators and particularly Binaltech, the larger size (and the diecast metal on Binaltech) really sell that premium feel. They're not really "toys," you don't really play with them, but the engineering is really impressive considering the limitations imposed on them by the licensors.
Alternators. You’ll never realize their play value until the kids start packing them with Lego minifigs as drivers.
I never had any Alternity figures…but I had just about every Alternator produced. I don’t think I’ve ever had so much fun collecting a line of TFs. They might have been the best bang-for-your-buck line of figures in the history of the franchise.
Alternity has some beautiful figures, but I prefer Alternators. Like @Zentropy I feel the smaller scale of Alternity held it back from achieving real greatness and also variety. Both lines wanted to create licensed, realistic cars, with opening doors and an interior (plus what else was possible), and have it transform. This is incredibly difficult. The robot parts need to come from somewhere/stored somewhere. The bigger you make the vehicle, the more space you have -- and the more space you need, but a bigger size gives more options for transformation. But as much as I prefer alternators, I do like the Alternity figures that I have. And it will remain a regret that when those were released KO makers didn't make OS KO's. Because I would love to have 1:24 scaled KO's of the Alternity figures. All of them.