I've not played highlander secret rogue, but I would say that putting secret package in highlander is a waste of time? Cutting your odds in half of getting the cards you need to synergise for secret rogue is spreading it just too thin to make it effective. I would say there are 5 better cards and more consistent to put in a highlander deck.
Full Galakrond is better in favored matchs like priest because you get the extra value more fast and consistent, highlander is better vs bad matchs because Zephyr and Alex can turn the tables in your favor.
That said, until nerfs comes the best is full Galakrond warlock, do everything rogue does but better and almost zero bad matchs.
u mean investing now in actual cards is better than investing in adventure in future?
No, investing now in the cards is better than investing in the Adventure. If you buy adventure now, it will last only for 11 months in the format. If you collect classic Rogue cards and cards from Ashes of Outland, they will be usable for much longer.
I am no Rogue specialist, but Highlander uses more cards, which will stay in the game after rotation. Galakrond and its support will rotate out in April next year.