It is hard to say how your benefits as a contractor will compare to what you will get at IBM. Some contracting companies offer benefits and some do not, in my experience.
Some things to consider before making the jump;
1) What is the morale like on the team you will join? A lot depends on who manages the team and who the 2nd line manager is.
2) Do you live in a "strategic" location for the org you will join? If not, there is a good chance you'all be asked to move so you can work from the office at that strategic location.
3) IBM has been in a pattern of poor quarterly results and subsequent RAs. Do you want to be a permanent employee at a company that looks at you as a cost and as replaceable at any time? While it is true that IBM will jettison you as a contractor on a whim, it's not like you would miss out on any severance package. If you want a permanent position, then there are many better companies out there than IBM.
I work on a successful team of very dedicated and talented individuals but I'm in my "lame duck" period at IBM having accepted a position at another company. Why am I leaving? Because I'm tired of looking over my shoulder every quarter to see if I'm on an RA list. I've been a 1 and 2+ performer for the last 6 years and I keep my skills current. I shouldn't be worrying about the axe falling, but there is no rhyme or reason to IBM layoffs and personnel policies other than cutting costs. I also don't live in a strategic location, and regardless of how well I and my team perform, we're not immune to this silly policy. I refuse to move my family. IBM encouraged work from home for years and now, at the drop of a hat, are telling folks, "Move or be laid off."
Even if you have a great management chain, they are powerless to save you from executive stupidity, and at IBM there are a lot of directors, VPs, and other pointy haired executives, so the executive stupidity level is always very high. Do you really want to join this circus?