Based on what exactly? Technical ability on it's own or actually putting your talent to use and winning matches, points, trophies and such for your team?
I'm aware that the statement itself is heavily subjective nature so I'm just trying to figure out your logic. To me, a statement of that magnitude can only be made based on the evidence to hand and the application of the player as a whole entity seeing as you've not specified a particular area.
Memphis is a year older, has played twice as many games, scored nearly 10x as many club goals - both in a weaker league granted but still, the application of the player cannot be faulted. He has also scored twice in the World Cup and played a significant part in his team finishing third in the competition. He is a league and cup winner, top goalscorer in the division and U17 European championship winner.
Januzaj has come through our reserve set up, won player of the year and broke through under Moyes. He had a handful of good matches and 1 match winning game vs Sunderland. He has had good individual moments in 45 appearances for United but on the whole, suffered badly with loss of form and effectiveness in 2014/15 season. 4 goals in total, 1 Community Shield, 6 caps for Belgium and that's about it.
On the small sample size, the evidence is in Memphis' favour so far, though that may change 3,5,10 years down the line. Even if you take trophies and personal accolades away, Memphis is still standing out. I'm wondering how you can draw a conclusion quite like that from the limited sample size of both players' careers to date.
But as said in the post before this, this thread is about unpopular opinions and that, to me at least, is one. Just trying to work out how you've made that call so far.