I don't think there's a huge gap between the Saga and the Venus, I own both of them and they play off of each other pretty well.  The Venus is the main ball I've been using on leagues with a THS this season because of it's versatility and how it smooths out the wet/dry reaction which I see alot.  If there's a little too much oil for the Venus, the Saga works for me because of the lower RG core.  If the lanes are soaked, I go to the Battle.   I think the Saga, Venus, Battle and a Spare Tire are a good 4 ball arsenal.  Just be sure to know what you're going to use each ball for when you go to get them drilled.

I haven't thrown a Horizon Solid (yet) but it'll probably fit between the Saga and the Battle in terms of reaction.  I don't really think you're going to have overlap between the HS and the Battle because the Battle is more for flooded conditions and the HS is probably more of a med-heavy piece.  I think the biggest thing in developing an arsenal is to have a plan for each ball in terms of what conditions you intend to use it on and have it drilled accordingly.  What you don't want to do is drill all 4 balls the same way because the difference won't be as much as you probably like.

For example, I'd setup the Venus for lighter oil patterns (weaker drill) drill the Saga for Medium, the HS for Medium Heavy and the Battle for flooded.  Just tell the pro shop operator what conditions you intend to use the ball on and he/she should be able to set you up accordingly.

For medium conditions I don't think you'll find a better ball than the Saturn, you can find a whole lot that cost more but this ball is awesome if you get one leave it in oob if you can its great and a super price as well
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Carl