Go for the Magicka, no question about it.
Honestly, I wouldn't even bother with the Mage though - go all out with the Atronach or the Apprentice. The latter is the one I like best. 'Weakness to Magicka' isn't nearly as bad as it sounds because it makes no difference to elemental magic damage, just to 'pure' magic effects, and the dangerous spells of that kind are very rarely found in the hands of hostile NPCs.
The birthsigns are indeed changed in BTB GI, and now the Atronach and apprentice aren't so tempting. In Vanilla Morrowind the choice was simple, High Elf with Atronach, the weaknesses of high elf would be eliminated by the absorb effect and I would have lots of magicka. But now the magicka multiplier of the Atronach is X1, the same as Mage and Apprentice. But Mage has an extra bonus besides this multiplier, 20 extra intelligence that can go over the 100 limit, so 120 intelligence maxed.
The Steed for all characters because moving like a crippled slug in molasses is no fun.
I've never been attracted to the steed birthsign, because I mostly travel by jumping around
. Its fun to jump over buildings eventually
.
You're forgetting that he is using BTB's mod. Consider that the birthsigns don't have the same effect as vanilla. Last time I checked, the Apprentice didn't have any weakness to magicka in BTB's improvements.
Anyway, the Mage sign is a good choice imo. All the extra magicka will be very handy in the future. Plus you can always use magic to improve your accuracy, if needs be.
Yes, the Mage is tempting, but if I have 30 strength I can't even carry my heavy armor and weapons. The Warrior besides the 20 strength that can max out strength to 120, has Fortify Attack 10, meaning 10 percent greater chance to hit, very tempting. But again, the Mage would mean 480 maximum magicka, while the warrior would mean 300.
So greater damage and hit chance, or more magicka? Still not sure.