iIcant tell you what the easiest test is for "pipe certification"  becasue its really dependant on the kind of pipe work your going to do......are you trying out for a FITTER or a Welder.....a lot of times the fitter must also pass a welding test for tacking purposes as well as a barage of skills/math tests.      If you are looking to pre-qualify for a welding test     generally you need a 6g in that welding process and material.  A 6g is a butt weld set up on a 45 degree angle you must weld in position.  This pipe will generally be between 1/4 and 1/2 inch thick and you will be allowed a bevel for a root pass and a filler and or cap pass.  Really the thickness is dependant on the kinda job your looking for.

I have done rediron work as well as pipe work......for most "Iron" work you will need a  structural cert which usuallly will be a 3g....Basically a 70/18 vertical weld on 3/8 to 1/2 plate.   The pipe test will be more difficult  but probably much more lucrative moneywise for you depending on where your at (amount of work to be had in each field). 

In Iron work you are liable to have to do structure welds and whats called "moment" welds  moment wleds are typically done with flux core wire welders and then sonically tested.  Being a welder (ESPECIALLY in union work) on a Ironworking sight is usually a nice job.  Guys drag your leads out for you, prep the weld area etc.   all you do is burn rods/wire until it gets slow....then you might get asked to go "bolt up" and other Ironworking tasks which are basically "HARD labor".

IN PIPEWORK  i welded a whole lot more but thats pretty much all i had to do was show up and weld.  Most of the time I had guys that cleaned my joints out for me and kept me supplied with what i needed.  I definitely liked the pipe work more.  Its just a bit harder to get certs and snag that work  but its worth the extra effort.  To know what you really need you need to know what company or jobline your gonna do to know what tests to take.

BTW.  A lot of the companies will pay to have you tested when you try for the job...saves you money and prevents you from wasting time and money on certs you dont need.

Good luck and let us all know what you ended up doing