I'd say make an AI3 speed deck (if you have NO cards for a deck, it should cost you about 1-1.5k for the whole thing). AI3 is best chance other than Top50 to spin a weapon.
I don't see the value in farming Top50 if you can't win most of the matches. You're just throwing electrum away, and you don't have a lot of it as a newer player. Consider these two scenarios:
1. You farm Top50 at 15 electrum a pop. Without a good deck, you're going to lose every match unless you find a farm. That means you will probably run low on electrum and have to play AI1-3 to win some back. You finally hit a farm, and you MIGHT win an Eternity (assuming that farm even has it). When you finally do win one, you've got ONE Eternity to show for all your time and effort.
2. You farm AI3 with a speed deck. You can probably win most matches with an unupped deck. Therefore, you are winning electrum and cards as you go along. Outside of Top50, this is your best chance to win ALL of the weapons, and AI3 speed farming is the fastest way to earn electrum. Yes, this process will probably take a lot longer to find an Eternity, but you will also probably win other weapons in the process, gain a lot of electrum, and get other cards that you can either use or sell for more electrum.
In summary: Top50 farms will get you an Eternity, AI3 will put you ahead in the long run.