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It depends on your size - if you are a small player, I think its better to sell vouchers during presale, buy DEC, wait a month, then buy the cards you want from the market or buy packs in future rounds that are not presale.

If you are medium sized or larger - it might be a good idea to do the presale for Tower Defense because 500 packs gets some pretty nice bonuses:

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For Riftwatchers its 1000 packs for a special title.

Thanks mate, good advice, I just know what ever I end up doing I will be kicking myself for not doing the opposite.