The title says: PL 9, so Europe has nothing to do with anything.
In terms of PL, Henry was to the PL what Messi has been to La Liga - a goalscorer; an assister; a dribbler from deep; a set piece specialist; the undisputed main attacking threat to be stopped at all costs (from the moment he got on the ball, usually just beyond the left side of the final third of the pitch) Henry, if you want to be pedantic, was never a #9 and actually wasn't particularly good as one either because it took all his best assets from him.
He wore #14 as a homage to Cruyff and he essentially played in a manner akin to him that started off on a nominally undefinable line between being a left-sided striker and a wing-forward whilst being neither of the two, but rather a pseudo player who masqueraded as one or the other at any given time. Henry started out as a left winger and learned how to incorporate his base knowledge whilst taking bits and bobs of being an off-centre forward, but never a proper central threat. He was impossible for centrebacks to pick up unless they followed him wide and he was rarely involved in any of the central striker shenanigans, firstly because he didn't have to be, but secondly, it was because it was not his game. In his era, he was often compared and contrasted with Shevchenko and Ronaldo who were equally adept at picking the ball up from deep, but much more natural going straight through the middle as traditional strikers than Henry was.
Why am I mentioning the obvious here, or preaching to the choir? Because "better #9" for all intents and purposes, has nothing to do with Henry, so much as to say, in the traditional sense of the word, he barely played that position or role - just as with Messi and Cristiano Ronaldo, you're hardly ever going to see them compared to the great strikers of their era because neither of them played that role. Even when Messi is central, he's a 'false #9' because hardly anything of the actual #9 role is relatable to his game.
The actual argument back in the day wasn't whether RVN was a better player than Henry, it was whether he was the better striker, or to put it another way: who would you pick from the two of them. It never really made sense to me, because, as stated, Henry rarely took up the positions or made the runs an actual #9 makes, whilst that's literally all Ruud did, barring a few uncharacteristic runs from deep, such as his infamous goal vs Fulham. In much the same way, comparing Aguero to Henry makes little sense when one is all about being a penalty box hotshot, and the other was doing nearly all his work a distance away from the box and would be loathe to scramble around with centrebacks looking for opportunistic scraps in the way a RVN or Aguero would.
Another way to put it, still, would be to ask who you would want on the ball in these scenarios:
- picking up the ball 50 yards from goal
- picking up the ball 40 yards from goal
- picking up the ball 30 yards from goal
- picking up the ball 20 yards from goal
- picking up the ball 15 yards from goal
- picking up the ball 10 yards from goal
- picking up the ball 5 yards from goal
- picking up the ball deep from the left side of the pitch
Anything less than 15 yards, and you're crazy if you say Henry, anything prior to 15, and you're crazy if you say Aguero. Anything at all from the left side of the pitch is a non-contest, just as 6-yard arts are the other way.
All these words just to say this discussion is like comparing an apple to an orange - both can taste great, but the only point they crossover is in being fruits.
Some would say Henry is literally the best player the PL has ever seen, whilst others will name him the best attacker, but in the traditional sense of a #9 striker: someone being fed ball through the middle of the pitch, from 30 yards or so and less, Henry has no dog in the fight as we're then in Aguero's territory comparing him with the likes of: Shearer, RVN, Kane and so on and so forth. Just as Messi and Cristiano worked with a centre-forward for the majority of their careers, so to could've Henry. Arsenal's set up was unique, but we saw for France what Henry looked like when he was more bound to the traditional #9 role, and whilst pretty good at it, the restriction of movement (he couldn't roam freely because Zidane had the attack built around him and he was the one playing free sometimes taking up positions Henry at Arsenal had all to himself) and the strictures of being a pure #9 seriously hampered and curtailed his game.
Final point to make is that the PL of Henry's era, as has been said, was a lot more cagey and defensive as opposed to the attacking free-for-all it is now for the league-winning sides of the last few years. Henry's goal tallies should be weighted against that because he was the one opening up defenses in tight games and he was the one supplying others, as opposed to the melee goal frenzies Aguero gets to partake in, which really do seem to be commonplace for City and their multi-pronged goalscoring threat.
I think Aguero's a true great of the PL era and a marvel at what he does, but outside of goal tallies, he and Henry have hardly anything to do with each other