Signal isn't a combat arm.
Here is the very short version of my opinion. With current operations, your best bet if you want a combat job lies in the infantry. The sad fact is, there aren't many operations where armor is necessary (nor even possible to bring for any number of reasons) and the same goes for artillery. Thus you'll find that with either choice, your odds of slogging about on patrol with the poor bloody infantry is quite high.
Aviation on the other hand virtually guarntees you a job in the air, but the catch is there is no guarntee you'll be granted the coveted right to fly something properly armed for combat. The bulk of Air Operations the Army conducts are transport and logistical duties, and as such you run a very high probability of flying a cargo or utility helicopter rather than an attack or recon helicopter.
Personally, when I was in the army (7 OCT 2003 - 6 OCT 2007) I was in the intelligence community, and this is the only job that I can speak on with any real authority. The field has several branches notably split into two distinct categories - collection and analysis. The collectors do exactly what one would expect: they collect intelligence. This may be technical collection of signal intelligence (The civillian agency that does the same thing is the NSA), or a more personal collection of human intelligence (source running, interrogations and so forth. the CIA is the civillian arm of this effort). The analyitical side of things takes the unprocessed data that results from the various collection efforts and uses it in order to predict future trends and actions in the enemy ranks (which is what I personally did).
Within the collection side, the huamn intelligence collectors are the most varied. Some of them are are assembled into small teams that spend much of their time "outside the wire", whereas others work in central locations dealing with walk-in sources and still others work at detention facilities as interrogators. I never once heard of a technical collection specialist being sent into the field regularly, generally because such personel are assigned to military intelligence units (which very generally provide services to a very large echelon of troops).
On the analytical side of things, your job is based almost exclusively on what sort of unit you are assigned to. At the lowest level, analysts are assigned to a Batallion's Headquarters company, specifically the S-2 section. These people are responsible for producing very, very specific intelligence - the precise location of an IED, the activity of a single insurgent cell and so forth. This is simply because a batallion is responsible for a relatively small battlespace and such levels of detail directly affect the comings and goings of the entire unit. The next higher level places one in an divisional posting and their scope widens dramatically. They are often concerned with general trends of activity across a very large portion of a theater and as such the specific things they worry about are on a much wider scale. The placement of an individual IED is unlikely to concern the activities of the division at large and as such they are often concerned with questions like where are the bombs coming from and where is a particual insurgent or terrorist organization going to focus it's efforts. At the highest level, one is assignes to the headquarters element of an entire corp or theater and again the scope of questions widen. Theater wide trends become important (new tactics, techniques and procedures across the theater, social and political considerations, tracking activity and predicting enemy activity that threatens the war effort as a whole, etc). I personally spent all of my time at the highest levels.
If you are interested in better details of the intelligence community, I may be able to assist, but there are of course specifics that I am unable to discuss.