To be fair given the physical shape Ronaldo's in, he could easily be a 30+ goals per season player till age 35. Reus in an undeniably gifted, world class player but I'm not sure he could replicate Ronnie's on-field exploits. On top form Reus could hover around the 20 mark but that just might be his ceiling. Ronaldo's such a greater threat not just from open play but set pieces aswell - something we've clearly lacked in recent years.
There's also something to be noted about ex-Dortmund players struggling from reproduce the ability/ production they previously displayed in Klopp's system and that is rather concerning. Out there Reus is almost the top dog but will he be the same alongside Rooney, Mata, Falcao, van Persie, Di Maria ? Wouldn't his numbers nosedive a bit ? Ronaldo in comparison would instantly command respect from those player going by his past record of being the Alpha even alongside some really top class footballers.
Off the field signing Ronaldo would have incredible ramifications too. He is an iconic cult figure among the Old Trafford faithful. He's a unique kind of player that could single-handedly reinvigorate the support and galvanize the club by his sheer presence. Not to mention the panic signing one of the 2 the best if not the outright best player in the world would create in the opposition's consciousness.
Would be like signing a 30 year old Keane or Scholes during our barren midfield era. If we have no chance of signing Ronaldo the it's OK. Reus would be the #1 choice of preference beyond him. But in a straight 1v1 head to head it's a non-brainer IMO.