Your buddies are quite wrong. A Bachelor's degree in business (finance would fall under business) will not earn them more money than you will with your engineering degree.
The business degree prepares them for management positions, but they will have to start at a lower position and work their way up, i.e. the degree will give them a huge advantage over fellow coworkers when shooting for promotions. Some business students are a bit more motivated and get unpaid internships early which they hold all through college and upon graduation have a management position waiting for them. But in this economy, paying internships for business students are virtually nonexistent. However, virtually every engineering internship is paid, and paid quite well might I add (they start anywhere from $10-$25/hr, a better "college job" than any of your friends will have!). I don't even have my engineering degree yet and my internship has turned into almost a full time job (made about $35,000 last year, part-time and still in college!) and they will be starting me at around $60,000 a year once I am a full-fledged engineer. A business degree will not guarantee as high of a starting salary.
Also, the demand for business degree graduates is elastic with the economy. The demand for engineers hardly ever diminishes to any noticeable extent. With your engineering bachelor's degree, you are guaranteed a job somewhere upon graduation, even without any internship experience. Show up to an interview with a bachelor's in business, and they'll tell you they've got a dozen MBA applicants ahead of you. You will probably end up settling for a job you don't like or lower pay than you expected.
I don't know what source you got your engineering salary information from, but they definitely do not "plateau quickly". In fact, most of the executives of manufacturing companies started as engineers. Business majors (especially finance and accounting) have little variety in their duties and often are stuck doing boring repetitive paperwork and mind-numbing meetings.
I know this is horribly biased toward engineering, but I am only answering your question as truthfully as possible. Several of my lazy friends left engineering for business and have had a hell of a time finding a job since graduation. But the key factor in success in any field is professional business and communication skills; I can't stress that enough. Pair an engineering degree with excellent communication skills, you're set.
If passion for your field is not important, stick with engineering. If you're mathematically inclined and properly motivated, don't waste that on a business degree. An economics degree will usually beat out business majors, but only if you are mathematically inclined. All my economics major friends are also at least minoring in math, most double major though and plan to get a master's right after their bachelor's. You would make as much or more with your bachelor's in engineering.
Anyway I've rambled long enough. If you're good at math, please stick with engineering. You will have a much higher and CONSISTENT income, and with your MS in Aero, you are golden. Even after a bachelor's in engineering, you can still go on to get an MBA or a JD (in fact, law school applicants who got their bachelor's in engineering actually have an advantage over most others!)
Don't give in to peer pressure! Your business major friends may be having fun partying in college now but your hard work in engineering now will provide a better life after college.
Whatever you choose, good luck and give it all you've got!