Aesthetics are what they are, and as you say preference here is totally subjective. For me, the nod goes to Siege Starscream without a doubt.

Let me say that I have not been collecting for about 9 years, after collecting almost everything for over a decade. I even wrote some reviews here back in the day. I left because I was burnt out, but Siege brought me back in. A great deal on Thundercracker had me grab the figure, and now I own 4 copies of the mold (well, 3, with Hotlink on pre-order). Because I was late to the party, I managed to snag a Siege Starscream with a good chestplate.

OK, so why Siege over Earthrise? The robot mode is first. Transformers for me is about the character first and foremost, and that has to come through on the robot mode. At a glance, someone who isn't a fan may not see a big difference, but for me, the small differences in proportion and in articulation make all the difference. The Earthrise Starscream has a long, boxy torso with no real waste and a skirt that contributes to the boxy look from the front, and a hollow look from the side. The lower legs are overly wide, and I don't care for how the tail fins contribute to the boxy look.  The Siege version has a tapered waist, and the torso looks good from any angle. The feet help extend the leg, making him longer, more slender. Toss in the added articulation - wrists, waist, and even wings - and when posing the Siege figure on your desk, he just looks more natural where Earthrise feels stiff.  You mention the Earthrise head is more cartoon accurate, but unless you are peering up close under good lighting, I don't think the difference between the two will matter much.

As for jet mode vs tetra jet, well, there's a long history of kibble issues with jetformers, and it strikes both molds here. Earthrise is a nice F-15 from the top, but underneath there's no missing Starscream's classic torso and legs. From the front, there's the hold in the chest. A lot of this was true of G1 Starscream as well, so maybe fans are just used to it, myself included.

The tetrajet pegs together very nicely and I was surprised how sturdy it is. There is definitely undercarriage kibble, but to me these are less obvious as legs and arms than Earthrise's torso is. The beauty of Cybertron modes is they don't exist, so it's hard to say the kibble is inaccurate, and it is really easy for me to see it as part of the tetrajet design proper (even if not G1 cartoon accurate). The only real annoyance is the chest plate on the back, but for some weird reason I like it. Either way, pick your poison: chest on back, or chest on undercarriage. Honestly I think it pretty close to a tie here for both figures, and both capture what they were going for equally well.

Lastly there's the transformation itself. Several WfC figures do things that take me by surprise, but that's not the case with Earthrise Starscream. Everything goes where it should, and it's fine but predictable, with a touch of irritation in doing the arms. As for Siege, I know it can be called a shellformer, but that's not how it feels to me when I do transform it. Things like the collapsing panel in the legs, or the nosecone turning into the shoulder pylons I found to be nice, unexpected touches. I really enjoy fiddling with the Siege figures, transforming them back and forth, morso than Earthrise.