I know this isn't what you asked, but if you want to get your 5k and 10k times down you really need to run more than once a week. There is loads you can do - tempo runs, longer runs, hills - all mixed up during a weekly programme, that will have a huge HUGE impact on your race speed. Come and join us on the Running thread. 
To answer your specific question, both yoga and pilates will help you if you are looking to improve your overall flexibility, core strength and focus on areas of imbalance/ weakness. They are also both good complements to running (which makes you tight in your hamstrings and calves).
Yoga is about movement with breath, and I would describe it as more holistic and inward looking than Pilates (I find it calms my mind, which isn't what I went to the class for, but just a great bonus). I find I get more from yoga; poses are held for much, much longer so you get a very deep stretch. You will also build strength, particularly as you get a bit more experienced and more flexible you will find you get deeper into poses and into more advanced poses too.
Pilates is more toning but also some lovely stretching movements. It is also focuses a lot on your overall posture, which sounds ideal for you with your tight pecs and rounded shoulders (sounds like me a year ago!!)
I personally think what will make the biggest different is finding an instructor that you love - my yoga instructor is AMAZING and I think that's why I feel I get more out of it.
Have fun whatever you do, and good luck with the running.