Welcome back to the skies!
Answer: get both!
Reasons: There is a lot of complex stuff, but you want options. To put it simply, if you pass all the exams etc for the NPPL, then adding a LAPL will only cost you a c£85 paperwork fee until Easter 2017.
(for fairness, you might also need a different medical)
After 2017, to hire a "spam can" (or to use proper technology, an EASA Annexe 1 aircraft - ie anything with a Certificate of Airworthiness), you will need an EASA licence: this is where the LAPL comes in.
To fly a homebuilt aircraft - your typical LAA types such as we have at Strathaven (come on down and have a look, you might be surprised!) and what most PPL holders can aspire to if they want to own an aircraft - then the NPPL is fine.
The other issue is keeping the licence/rating current.
With the NPPL, it is valid for 24 months. You have a certain number of hours to do, some of them must be in the second year of validity, and a minimum 60 min instructional flight.
To use an extreme example, once you NPPL is valid, you could not fly for one year and 364-ish days and then go for a legal flight.
For the LAPL, there is a rolling validity. So you have to check before each flight that you have the 12 hours in the past two years and the hour's refresher flight with an instructor.
From the CAA:
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To be able to fly you must have completed, in the two years before any intended flight, a total of 12 hours as pilot in command of an aircraft covered by your LAPL(A) privileges in addition to 1 hour of refresher training with a flight or class training instructor.
If you haven't completed these 13 hours in the last two years, you must rectify the situation before flying again under your own privileges.
If you are merely lacking the hour of training with an instructor in the past 2 years, you are allowed to complete that task to restore your own personal validity (assuming you still have 12 hours pilot in command time in the previous 2 years by the time you wish to fly again).
If you lack some of the pilot in command hours in the previous 2 years, you can either opt to complete a proficiency check, and take a flight test with a flight examiner, or you can build your solo hours as though you were flying as a student pilot in command. To do this, you'll need to contact an ATO and be signed out for solo unaccompanied pilot in command flights by a qualified instructor within the ATO. In practice this may require some dual flights before the instructor and ATO is willing to authorise the flight to start building your pilot-in-command hours. Once you have built sufficient hours in the previous 2 years to satisfy the LAPL validity rules, you can once again fly under your own privileges.
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What the CAA don't say is you can use your NPPL to get the hours to revalidate your LAPL!
Take the situation above, where you had completed all your NPPL revalidation stuff three months before the NPPL was due to expire. Then you get your NPPL reval signed off so it is valid for two full years from its expiry date.
Over the next 25 months, you only fly 11 hours in a light aircraft, so your LAPL is now invalid. You either need a proficiency check or a supervised solo flight - says the CAA. But you also have an NPPL, so you go for an hour's bimble using that and your LAPL is valid again!
Well worth the £85-odd extra, I would say. Although if you really were rusty, an hour or two with an instructor might not be that bad a thing!