Experience with both systemsI have owned both an AW 5500 and a PB 1.5. The alienware died so I decided to try a mac. Now I'm not going to tell you what you should buy, because ultimately its your decision. It really all depends on what YOUR going to do with it, not what any of us do.
But I will give you my experience with both systems.
With my alienware, I was a gaming freak. I could play any game that was out and that was fine. Then my harddrive crashed. AW replaced that for me and I was back up and running in about 4 days. That was the 1st week that I had it. I had a year of good running time on it, and then after I moved to China to teach English, my AW died. Just wouldn't turn on. After much troubleshooting, I called AW, who told me that they could fix it for me, if I could get it to them. Well, I couldn't. So I was without a computer for 4 months.
During this time, I started looking at Apple. I bought a 17" 1.5 PB and I've had it for about a month now. Does it play Doom 3? No. Does it play almost every other major title to date? Yes. Does it play those games at the highest possible frame rate? NO, but my Alienware didn't either. Especially since you said you are playing RTS, you do need to worry about the video THAT much. The new PB have a Ati 9700 in them, with 128m ram. Thats what the 5500 has.
A lot of people have commented on the screen res. I admit the 15 is a little low, but the 17" is just right.
Another consideration is weight. I find it quite funny that people say weight doesn't matter. When I owned the AW, I said the same thing. "Oh it doesn't feel heavy." Yeah Right! When I first picked up the 17" PB I was amazed at how light it was. IF you want a LAPTOP, weight matters. IF you want a "Mobile Desktop" then weight doesn't matter. But you will never have a true "mobile desktop." Becuase a laptop will NEVER match a laptop of the same configuration.
Then you have to consider that you have to deal with Windows if you go AW.
Mac OS X is the best os ever. No viruses, no malware, you just don't have a to worry about all that stuff.
Wow, long post, wasn't expecting to write that much. I'm not saying you won't be happy with a 5500, but I know you'd be happy with a PB.