At level 30 this could be:
swordmage:
10 base
+15 level bonus
+11 armor bonus (+6 elderhide armor)
+2 feat bonus (greater swordmage warding)
+8 Int bonus (18 starting Int, improved at 4/8/11/14/18/21/24/28)
+3 swordmage AC boost (one-handed blade w/nothing in other hand)
= 49 AC
sword-and-board defender in plate
10 base
+15 level bonus
+20 armor bonus (+6 godplate armor)
+2 shield
+1 feat (either armor spec or shield spec)
= 48 AC.
Swordmage can go higher than that:
10 base
+15 level bonus
+11 armor bonus (+6 elderhide armor)
+2 feat bonus (greater swordmage warding)
+10 Int bonus (20 starting Int, improved at 4/8/11/14/18/21/24/28, +2 Epic)
+3 swordmage AC boost (one-handed blade w/nothing in other hand)
= 51 AC