If it was me personally I would spend it on the champions bundle, Since your new and don't really have any champs I would think it would be the best bang for your buck. They are all older champions but most are still very viable today. You get a nice mix bag of ranged adc, support, AP casters, Junglers you name it. 20 Champions for $25 cant really go wrong with that! I wish I did it when I first started playing.. but since now I own like half the champions out of each different bundle I wouldn't get my moneys worth.
Hoped this helped and Welcome to the League have fun =)
Oh and I almost forgot, I'm not sure if its still going on but back in the day if you subscribed to league of legends on face book you used to get Tristana for free! She is still one of my mains for adc to this day =)