The largest difference between Space Empires V (SEV) and Sins of a Solar Empire (Sins) is content. SEV has almost everything you can include into a 4x game while Sins has much less.
In SEV you've got stars, planets, asteroid fields, black holes, nebulae, dyson spheres and ringworlds, planet creation and destruction, sun creation and destruction, etc. etc.
In Sins you've 5 types of planets and 3 types of asteroids, 4 types of suns. No planet creation and destruction (destruction may be possible, creation surely not).
In SEV you can outfit your ships yourself and there are lot's of different things to outfit with. Although most of the equipment doesn't make very great changes how battles play out. There are also mines, drones, fighter/bombers and carriers, star bases, ground troops. You can capture ship, destroy every equipment in ships separately with special weapons. The battles themselves are in 2D and quite weak with a bad AI.
In Sins you only have three categories of ships (frigates, cruisers and capital ships) which can't be customized much. But ships have very different role, making very different battles depending on ship type. Also combat is more WarCraft like with "spell casters" ie. ships with special abilities.
Generally speaking, battles are a much higher priority and a larger part of the game in Sins than in SEV.
As said above, Sins has a very small research and diplomacy part compared to SEV. The research has much less impact than it has in SEV. Your ships get maybe 50% better trough research. Not the 1000% increase you can get in SEV through larger hulls, stronger shields, heavier armor and new weapons. Also there is no espionage in Sins.
But of course you can't just compare content. Implementation is also very important. Here Sins is much better and more polished then SEV.
There are almost no bugs in Sins. The AI in Sins is not very good, but a hundred times better then the AI in SEV. The AI uses all things that are available in Sins.
Sins is quite balanced atm. compared to SEV.
Sins is real time, thus much faster then SEV. Still, Sins matches can to on for hours after hours. Sins user interface is clearly superior to the SEV interface.
In the end, Sins and SEV are too different to be really compared. SEV plays in the hardcore 4X category and content wise it shines. There is no other 4X with so much possibilities available. Sins on the other hand plays much more in the RTS genre, with real time tactical battles and all that.
