Some Swordmage powers are designed to be used with Aegis of Shielding, and they're better. The top of the Aegis-specific powers is Transposing Lunge. It's the first power I've seen that I'd be willing to keep on a character all the way from level 3 to level 30 - it's just that good.
Better?
Aegis powers like Transposing Lunge have a weakness if taken. The AoA Swordmage does not need (or even want) to hang onto his encounter powers until his mark triggers like the AoS Swordmage who takes Transposing Lunge. So, the AoA Swordmage can use his encounter powers earlier in the encounter to help decrease the number of foes, hence, shortening the encounter. He doesn't care if he teleports and uses an AoA encounter power, or a basic melee attack. The AoS Swordmage player cares and will more often defer using Transposing Lunge until he can use it with AoS.
There are more AoA feats (4) than there are AoS feats (3). There are more AoA encounter powers (5) than there are AoS encounter powers (4). Granted, a PC could put a lower level AoS encounter power into a higher level encounter slot, so all encounter powers could be AoS powers, but that is not a good strategy.
And, there are Swordmage encounter powers as good as Transposing Lunge. Corrosive Ruin is as good or better in everything but solo encounters (and is considerably better in minion encounters compared to Transposing Lunge). It can easily average as much damage as what Transposing Lunge does even if one combines the amount of damage Transposing Lunge does with what it saves.
And a major difference is that the Swordmage is a Defender. He is often out in front. Damaging a bunch of foes early on in the encounter with Corrosive Ruin is often better than waiting for a marked foe to eventually stop beating on the Swordmage and have it beat on someone else and then use Transposing Lunge. I would expect there to be combats when Transposing Lunge is never used or not used until near the end of the encounter, just because the player realizes that its AoS function will not trigger. I wouldn't expect that as often with Corrosive Ruin.
Yes, Transposing Lunge is better in solo encounters. But, solo encounters, at least IME, are a lot more rare than non-solo encounters.
The significant advantage of an AoS Swordmage is that AoS is an immediate interrupt, not an immediate reaction like AoA. The AoS encounter powers are not what makes an AoS Swordmage good. They can be as much a detriment as they are an advantage.