Long Term Strategy for Getting +5000 trophies

In my opinion, the 2nd strategy is much better.

Now why is it so?

1. Using just one deck will give you the advantage of higher card levels, which is way better than just better KT level, for example, I have been using arrows in almost all of my decks, even in arena 10 now, and I have this card on level 11, so I can one-shot lower level archers(which is surprisingly common) with it.
2. Upgrading everything possible can leave you with no cards, unable to donate them, as donating is a more efficient way of leveling up your King's tower(if you are not a pay2win player).
3. Again, if you are a free to play player, it is better to save money, so that you can buy expensive cards in the shop that you will play almost any deck(like Legendary cards), so you can upgrade them. By donating, also, you earn not just XP, but even in-game gold.
4. Using a deck or deck type for a long time also gives you the advantage of skill when playing this deck type. If you change your style, it takes long to gain skill for that deck, so, it is more beneficial to upgrade cards of just 1 deck.

Yeah. Always when I meet a level 9 KT with level 12 or 13 elite barbarians or Royal giant, It is harder to beat than someone with KT level 11 or even 12 with tournament standard levels of cards.

Hope that helped :)