What card can you buy and use right away? There's no point spending it on Leviatonn if you don't have the cards to go with it as it'll be some time before you can enjoy using him.
Looks like you have the Junkz to back up your Gil or Rowdy, so I'd go for one of those. Use them in Elo or DTs and save up for the other decks.
I'm more of the opposite with this as Gil when used on a card you expect pillz on can be a devastating blow to your opponents pill reserve, he lets you over pill a bit ( Yes there are times when I play second I'll drop 7-8 pillz on him and still win) because of his ability and bonus. ( For some reason some people seem to really want to win against him, and will at times pill 5-7 against him, on the other hand you could just pill 3 and if your opponent pillz zero your equal or you'll have pill advantage because they did pill)
One of the best hand combo's to me with in the Junkz clan is Gil /Tremorh as then you can have complete control only with an attack mod to boot. ( You can even bluff Gil in this case, thus getting you a pill advantage then allowing you to lower them with Tremorh after word, and make them pay for that mistake)( Soa clans affects this plan a bit, but soa abilities you can work around)
Rowdy is a great card but he is also a 4 star, and Gibson can be a great filler for that spot until you can afford him. Malmoth is another cheap option that is great because its dual manipulation which can save you in lower pill fights or higher pilled fights.
Gil is a 2 star, and allows you to fit in Peeler, Haze, Fuzz, Taham, DJ Korr Cr ( Tho I figure those that have him also probably own the whole Junkz clan) , or my personal favorite Hugo while not being a filler you have to throw in there.
Gil can also be a splash card if you want pill manipulation in a deck.
Truth be told, you should be fine either way. I would get whichever card you personally think is "cooler". More than likely, you will play better with whatever you are more comfortable with. Further, there are viable replacements for either until you can save up for whoever gets left out on the first go.
Me? I would probably pick up Gil if I had to choose. Being able to fairly consistently seal the game by the first turn is pretty powerful.