It is a pure delusion IMO to lift endurance style and expect to have a visible lean mass and definition. It is simply not going to happen!
My results dispute this. I have spent the past 6 months doing almost exclusively endurance weight workouts. This, coupled with a focus on upping my fat and protein intake, has led to amazing results. I would normally not be so outspoken about my physical appearance because I primarily workout for the health and mood benefits, but to illustrate my point I have to say that I have never ever looked more cut than I do right now, both in my upper body and lower body. (I have been married for almost 13 years and my DH told me last weekend that I look more buff than ever.)
I will say that I think the biggest factor has been nutrition (eating more good fats and increased protein). My weight hasn't changed (I wasn't looking to lose weight), and I have not changed the frequency or intensity of my workouts. I've been working out with Cathe's DVDs for 9 years. Based on my experience, I can tell you with 100% certainty that I have gotten awesome results from endurance workouts.
I firmly believe we are all different, and that what works for one person will not necessarily work for another. Also we all enjoy different types of working out, and like Nathalie, I plan on a Body Beast rotation soon for some variety.

So, LFleming, I say go for it and do what makes you feel good and with what you feel gives you the best results. You said you're doing STS, but mostly arms...have you done Meso 1 legs and Meso 3 Plyo legs? Those are awesome endurance style workouts. Plus Cathe has so many other endurance-style DVDs for the legs, like Lean Legs & Abs, Great Glutes, Cardio Leg Blast, Lower Body Blast, Legs & Glutes (which is a lot like a Meso 1 workout), and Leaner Legs. Often I will have weeks where I do 2 leg workouts (like Plyo legs on Monday and LL&A on Thursday), or where I do 1 leg workout, 1 total body, and 1 circuit (plus 2-3 days cardio). And I agree with RapidBreath that kickboxing is awesome for shapely legs, and I also love spinning!