All training is good, and without looking I don’t think training has an expiration time limit.
MT is a good place to start, if you can afford the training id say take it.
MDK

I also think the training hours is valid for life, I have a friend who just completed UT level II 15 years after taking level I (LOL)
He agreed that it's bad idea to wait that long.

Since you are doing well in your new CWI career (I assume so since you have been 8 months the same place) why don't you go to the next step....coating inspection, API, classes about PWHT, materials, etc...Expand your knowledge a bit.

"Soon" you will find yourself in a rewarding job position, which often is rewarded with a nice check each month :)

Good luck with whatever you do....

3.2

If I were to consider A training  program to take I'd consider concrete construction inspection,MT, and NACE certs
this is why. I worked for a company that made bridge bearings (they moved and I opted not to go family), I still talk to them occasionally and they tell me that business is off the chart. Id bet that 25% of the bearings for Rehabs and new construction is coming out of that place. So, with that in mind I would assume that concrete construction inspection would be good as lots of these bearings sit on concrete.
Then id consider NACE(painting/coating) because most inspection agencies I've dealt with would always want to grab the whole inspection contract . So to get your foot in the door with a good agency get NACE certs,CWI,Concrete certs,and MT training.
MT is more likely the ceapest id do that 1st
http://www.hellierndt.com/magnetic-particle-testing-levels1-2.html

But i always think about where do i go if the current situation no longer suites me, and I know there is stimulus money for lots if not tons of semi long term gigs out there in the bridge relm.
http://events.nace.org/education/certification/cip/CIPbrochure07.pdf

MDK

Based on your post, I am assuming you obtained your CWI through welding experience.

A few considerations for you first:
The CWI is no different than the welding. You learn new processes and techniques to expand your knowledge and marketability.
With that said, your thinking along the right path, but you should consider what industry or industries it is you wish to work in.
You should also allow for the fact that nearly all industries ebb and flow over time, and if you tie yourself to just one, your going
to find yourself tied to the inherent slow periods of that industry, and not just the highs.
Most of the major industries can also represent facets of others.

I break down the major industries as follows:

Energy: Petroleum, Nuclear power, fossil power, solar power, wind power, tidal power, hydro power etc.
Mining: Minerals and metals and all the associated apparatus to get it out of the ground.
Naval: Surface ships, Boats, and nearly anything that utilizes water as a means of transporting either civilian goods, or military platforms.
Aerospace: Anything from Cessna's to Moon launches. If it takes flight in any shape form or fashion by design, it qualifies.
Civil: Structural buildings, Bridges, stone work, concrete work, factories, any and all things related to occupied structures, and some non occupied.
Military: In a general sense, anything related to military devices not directly connect to aerospace or naval. (mortar shells, bombs, guns, rifles, tanks, etc)
Ground Based Transportation: Cars, trucks, trains, and anything that utilizes dirt, rock, rails, etc for a means of travel.
Chemical: A broad heading to mean any manufacturing

You'll note that most of the major industries are so intertwined that clear definition is sometimes hard to do, but there are in fact only 8 major industries in the world as far as an inspector is concerned. As a general statement, nearly everything will fall under one or more of the eight listed.
My personal experience is heavy on Energy, Military, Naval, and Aerospace. I've been in aspects of them all, but more so those four.

With that said, the question in regards to mag particle usefulness would be dependant on which of those general headings you wish to work under.
As an example; Mag particle is not as useful to aerospace as it is to some of the others. API certs usually mean nothing to the Mining and in most cases naval, civil and transport.
For that reason, I would suggest getting a more generalized cert if your going for something else.
Your CWI has application in all of them.
NACE certs are utilized in all of them.

For NDE it has to be broken down into probabilities. NDE as a whole is utilized in all of them, but just as is with welding, not all methods/processes transfer very well to all of them, or have limited transferability. Below is a list based on personal experience and observation, with ratings from 10 (Used in all industries heavily in one form or another) to 1 (niche market, used less)

Ultrasound 10
Radiography 10 (for now, but changing soon)
Penetrant 8
Magnetic particle 8
Visual 10
acoustic emissions 5
leak testing 7
nuetron radiography 1
eddy current 6
thermographic 4 (but growing fast)
vibration analysis 6

If you go for an NDE certification, go for one with a 10 first, then down from there, but don't bother with anything less than 7 if you want the maximum transferability.

My opinion for what it's worth,
Gerald