In the short term (1-2 seasons), Firmino IMO. The Bundesliga is much better in terms of quality than the Eredvisie on a weekly basis, and he has been one of the best offensive players in the league for about 2 seasons now, going against the likes of Bayern and Dortmund and Wolfsburg and Monchengladbach. One might counter-argue that by calling Memphis the best player from Eredivisie last season bar none. But he probably won't be as prepared as Firmino through the adjustment period in terms of adapting to better opponents on a consistent basis. And although Firmino is by no means the final product, he might prove to be more consistent at first because his overall play does have more polish to it because of his greater experience, age and even physical maturity. So he can pretty much be a day one starter, link-up with Coutinho, and perform at a good level whereas Memphis is likely to have his ups and downs and will be eased gently into the team according to Van Gaal (so less opportunities to shine).
Medium to long term, I think Memphis will prove to be the better signing because he's simply the more gifted and focused individual, and is arguably one of the 3 most talented young (U-21 sorta) offensive players in Europe - particularly when you consider his year-to-year developmental jump. He will have to pay his dues at first; the gulf in not just quality, but pace and physicality and endurance between the Eredivisie and Premier League is significant. He might be buff, but the league does take its toll on young players. But what bodes well for the future is his well documented determination to be one of he best, and his an insane Ronaldo-lite type of work ethic. Wouldn't be surprised if he becomes one of the absolute top players in the league or even Europe once he finds his groove. With Firmino I'm much more cautious. He has the potential to be a very productive player, but he most likely won't be one of the top top players around (like ever).
