X-Plorers. Multiple times a day for the past week. So, here goes nothing ;)
Presentation-wise, White Star wins hands down, and I'm not just saying that because +Jason Paul McCartan is a good friend of mine. X-Plorers has a serviceable layout.
White Star evokes the feel of Star Wars, Firefly, Guardians of the Galaxy and even Battlestar Galactica. X-Plorers feel evokes the movies and TV series of the late 50's and early 60's. A big portion of that may be the cover and the Earth-centric setting (if four paragraphs a setting makes - and to think I thought four pages was limited to work with ;)
White Star uses the six attributes you already know from the OSR game of your choice. X-Plorers reduces that number to four. Wisdom is rolled into Intelligence and Strength and Constitution are rolled into Physique. Charisma is renamed Presence and Dexterity is renamed Agility. Why? No idea. White Star defaults to a -1 / +1 spread, while X-Plorers defaults to a -2 / +2 spread on attribute bonuses.
X-Plorers uses a universal experience table whereas White Star has an experience table for each class.
Both games have 4 base classes each. In White Star, they are: Aristocrat, Mercenary, Pilot and Star Knight (as well as 3 other classes - Alien Mystic, Alien Brute and Robot). In X-Plorers it's Scientist, Soldier, Scout and Technician. Pilot and Scout fill the same niche and Mercenary and Soldier. Classes in White Star have "class abilities" (think AD&D Paladin kinda sorta in the way it's handled) which are like minor powers or boosts and X-Plorer classes have skills that are rolled on a D20. This is a very major difference between the two games.
X-Plorers includes rules for multi-classing, which is pretty nifty, but it isn't really true multi-classing - it's learning skills from another class. White Star doesn't have any such rules. It might make for a decent houserule.
I'm surprised that X-Plorers doesn't use the dual stated AC from Swords & Wizardry, as it builds S&W. Not a big deal, or even a little deal, just interesting to note. White Star uses the dual stated format.
Equipment in the two games overlaps at points, but either list would make a good supplement for the other's game. Armor is much more expensive in X-Plorers, by a factor of 10 or more in some cases. It is also more generic in White Star, as it is described by light, medium or heavy instead of Mesh Suit, Reflec or Combat Armor in X-Plorers.
Part II later today.