According to the report of others here, the stats page on the Banu is completely wrong. Apparently two of the S3 mounts are not guns but pylons, which means good missiles. Some of the Vanguard carry S3 missiles, others do not, so you need to specify which Vanguard when comparing.
Also the ships page makes it appear the Defender has crappy shields, but someone reported it has good shields. The Vanguard has excellent shields with a very high refresh rate.
In any case, the Defender cannot mount S4 and S5 guns. I happen to like the bigger guns, because I like the advantage of striking from out of range. That is one of very few advantages you can plan ahead on. Same with carrying S3 missiles--if you opponents are limited to S2, you can hit them before they can hit you, which is BTW the primary combat strategy of all Fifth Generation fighters--fly into range of your missiles, lock, fire, fly away before you can be locked. Its a good strategy in general. Though we do not know what the missile lock system will be like on the Defender, it probably does not lock as quickly as the Vanguard, as CIG planned the Vanguard missile lock system to be a superior item to others.
If I were flying S3 guns in a fighter, I would be strongly tempted to fight from extreme close range and mount all Pyros. That strategy may work well for the Defender. If so, it may be the Vanguards and Defender develop this distinction, that the former habitually fight from great distance where their guns are effective and the later habitually closes on an enemy. Both strategies have great merit. Once a close distance brawler closes on you, generally the only way to get away is run, which ends in the brawler chasing his opponent from behind. If the Defender can keep pace with his opponent he will have the tactical advantage once he closes the distance.
OTOH, the Vanguard would get free shots. If you hit with your first shots from outside your opponent's range, you can end a conflict before you are in danger. If however you miss, then they can be on you and the Vanguard does not dogfight well.
So this is my best guess, if you like scraping at close quarters, you may like the Defender better. If you like hitting when you can't be hit back, choose one of the Vanguard.