To be real honest I am bias towards 15mm.
I would love a line of "Official" Traveller figures. A complete line. Then has never been one.
If I where to start producing them I would start with a pack of Hivers, then the rest. There has only been one Hiver figure ever produced, that was by RAFM for TNE. Not a bad figure, just rare, it's harder to find than Grenadier's Imperial Marines.
As I said my preference is 15mm. This is partially all the old Large lines of Traveller figures where in 15mm. Vehicles don't cost an arm and a leg in 15mm. And ranges in running gunfights aren't ridiculously short. i.e. vehicles don't look clownish on the table.
As mentioned before most of the Citadel/Rafm 15mm figures are available. Ground Zero Games' 15mm figures match well with them. GZG's UNSC Hardsuits are Imperial Marines. And they also make several packs of Vaccsuited figures. Not to mention their vehicles, a very nice Air/raft, as well as several flavors of Grav Armor. I would use the Advanced Grav series of armor as Imperial Marines, and the Grav Sleds as Sword Worlds.
But if I where going to choose another size 25mm would be it. The size of the Grenadier figures. The reasoning is that with 15mm. 20mm bases are viable.
That is my 2cents on the subject.