Invest in L lenses and never look back. Don't waste your money on 3rd party lenses.
Now if you hadn't given this as an advice but as a statement of your personal preference, there would be nothing to discuss about. But as it stands it's just the type of very bad advice that is frequently found in this forum and any shop would give you if they would want to give as much money out of you as possible. For aps-c (and not the op's mk2) its even more complicated, because part of the expensive glass ef lenses is not even used for this sensor type, thus it's even more strange to recommend L lenses for everything
* to the op: You can pretty much decide for yourself since you've good one crappy lens (50/1.
and a good one (24-105L). If you can live with the 50mm Canon one, you'll never find anything wrong about decent 3rd party lenses. But there are excellent 3rd party lenses out there, For crop Sigma or Tokina ultrawides, the Sigma 50/1.4 is better than the legacy Canon one, and in the tele range some Sigma 2.8 lenses outperform cheaper Canon ones hands down. Even in the af/usm/is departement, the 3rd party manufacturers seem to have caught up. So you won't get around comparing reviews and prices to make a decision, or of course if weight or budget is no matter get something like the 24-70ii and the 300 and up 2.8 Canon tele primes.
* personally, I've always bought Canon lenses because of the higher resale value if I would ever want to switch brands completely. As for the other general differences:
a) One constant annoyance about 3rd party lenses is quality control, so be sure to test a new lens and get it replaced if it shows sharpness falloffs or unbalanced CAs.
b) you might need more af micro adjustment on 3rd party lenses, but your mk2 has got this feature.
c) 3rd party manufactures allow for longer warranty than the very short Canon 1y.
d) 3rd party lenses might have issues with upcoming bodies (i.e. not your mk2) because on the one hand side the protocols are only reverse engineered by some and on the other hand side Canon seems to make sure that there are problems.
e) Canon only delivers in-camera correction profiles for their own lenses, but you don't need these if you shoot raw and then apply a profile in dxo or lr.
f) Canon L lenses have an annoying red ring and even more "here comes the money" white color which might give you a short ego boost and device the crowd in front of you, but later on imho are rather embarrassing (I color-taped my big white lens to get around this).