I worked as a IRS IT specialist for a couple months on a detail, and still work for the federal government.
Pros: Federal benefits are great. Lots of leave, alternate work schedules, decent insurance, and great retirement. This is true of any federal job. It's also very stable, particularly with the IRS IT position - they are going through a huge modernization effort now so you will definitely be busy.
Cons: As far as your salary goes, yearly grade increases are not guaranteed, and subject to time-in-grade requirements. If you are starting at 38,000 you would probably be a GS 7 now. It will take you at least 3 years to get to GS 11 (55k ish), assuming everything goes well. GS 12+ are usually promotional (ie you have to apply and interview again), and likely are managerial. You won't hit 100k until GS 15, which is usually for people with 10+ years of service. So I would take the 100k promise with a huge pile of salt.
Another con is that everything moves pretty slowly in government. The onboarding process will be slow, training will be slow, and getting new learning opportunities will be slow. Unless you have an excellent manager, there is really no way around this.
Personally I like working for the feds because of the stability, leave, retirement, and opportunities to transfer around between agencies. I also really like my manager and co-workers.
But if salary, a fast-paced and adaptable environment, and keeping up with the private sector are important to you, I would take the other job.
Feel free to PM me with questions about working for the government.