((SEIV is probably easier to modify, and seems to be vastly more popular. SEV has real-time combat, a different approach to intelligence, more restrictive race design, less restrictive racial technology(IIRC) and various other changes. I mostly notice the theoretically three dimensional real-time combat, it seems to make most things easier on the artificial intelligence. Other interesting differences that I am recalling are the ability to retreat from battle, hex-based system maps, moons and planets cannot transfer cargo, Carrier Battles-style armour(with a percentage chance of hitting the armour or a component) seems to be impossible, systems are sort of roundish instead of square, battles are very roundish instead of square, ground battles are depicted and can involve fighters and defence platforms(but I think they are square-mapped...), the AI options are somewhat different, research allocation is slightly different, scanners have limited range...
