Ii'm going to pick a nit, here: neither of these are "lugged" forks. they both have the same forkcrown which is an external socket investment-cast type with smallish reinforcing blade tangs (looks like the Long Shen LC-15). The difference between the 2 is the blade construction: Infinity is a seamed tubing, so probably has seamed fork blades, where Prestige is seamless...according to the specs offered they both have the same rake, I'd assume the Inifinity weighs slightly more. Main diff is that the Prestige only comes threadless...that's important.
The "advice" your LBS gave about an aluminum fork needing replacement after 7 years smacks of over-caution combined with salesmanship: it's true that aluminum does have a limited lifespan compared to steel, It's certainly going to degrade after years of stress, but unless you actually inspect and test like an airframe inspector would an aircraft, 7 years is just a guess. But I'd rather have either of these steel forks over an aluminum fork any day.