No, you won't disadvantage yourself. Conversions are seen as equivalent to LLBs for the purpose of TCs. I am a current trainee at a MC firm and assure you that people with a full law degree (like myself) are definitely in the minority (but only just, its something like 55% GDL 45% LLB). I know other trainees who did their undergrad in the US and loved it.
The only real disadvantage is that you don't have the opportunity to study things like Family law/International law on the GDL, but as you have already decided you want to go to a commercial firm this doesn't really matter.
My personal view is that you should go to Penn if you prefer it and the cost is taken care of.
If you had ambitions of practicing law in the states (apart from the headaches with citizenship), then you would want a full law degree as it is generally required to be admited to practice in the US (in fact most states require you to have completed a full JD in the US). There is a real cultural difference; conversions are completely accepted in the UK but not in the US.