79 vs 87 S4 ?

Without knowing your reasons for owning a Porsche 928, I don't know how to answer your question. They are very different cars in terms of technology, performance and even appearance.

1. Is future value a goal? Some purchase Porsches as a financial investment. Some very pristine early 928s (and newest GTS models) are selling at a premium and may continue to increase in value.

2. Driving? The early 928s have single piston floating brake calipers vs. S4 (well 1986.5) and newer 928s have four piston Brembo calipers for more capable and lower brake fade driving. The 16-valve early engines are 4.5-liter displacement produce a bit over 200 HP, the 5-liter 32-valve S4 produces a bit over 300 HP, the difference in power is quite notable. Even though the early engine produces less power, the S-4 can get better fuel economy as the engine management system (spark and fuel) is much more advanced.

3. Appearance? Some like the original design (purity/originality?) some like the updated (more modern?) front and rear of the 928 S4.

4. Maintenance history? The S4 has "newer features" so there is more to go wrong. The 16-valve US engine is thought to be non-interference so a skip or break of the cam belt isn't catastrophic, whereas the valves in the 32-valve motor will contact the piston and bend if out of time with the pistons. Both K-Jetronic CIS fuel injection and LH fuel injection have their unique maintenance issues.

I own a 1979, 1987 S4 and 1993 928 GTS. Each car to me has a different personality and I pick them for different "missions" on the road. For me, owning both this the answer.  I hope this information helps with your decision.

Learning that you are a Porsche enthusiast/collector and own a 356 I can understand how acceleration performance may not be a top issue for you.  The early 928 is what was Prof. Ernst Fuhrmann and his team developed to save Porsche at the time because they feared the 911 couldn’t meet future safety, noise and pollution regulations. You ask a difficult question.  I didn't include my 1993 GTS in my initial reply because that wasn’t a model in your consideration.  I purchased my Amazon Green GTS (in my avatar) new. 

 

It is my pride and joy and lives a pampered life.  It is the culmination of the 928 as a GTS car.  The power is superior to all, the interior is more luxurious.  I installed a Porsche Classic radio to add some modern car features.  Some may consider it sacrilege, but I sought out an automatic transmission during my purchase.  The GTS has so much torque it normally starts in 2nd gear.  On a lengthy drive, the automatic transmission shifts better than I do with a manual.  So, I would keep my GTS. However, I actually prefer my 1987 S4 for road trips.  It now has nearly 188,000 miles on the odometer, 60% of which I have driven since ownership.  It has only had good maintenance and a transmission rebuild but the engine is original with no appreciable oil consumption.  Some complain about the 2.2 : 1 final drive ratio of the differential, but I find the lower RPM at cruising speed to be preferable for lower noise and longer engine life.  In comparison, the GTS has 2.538 : 1 (manual 2.727 : 1).  For reference, the 1979 manual has a 2.75 : 1 final drive ratio.  

My 1979 928 is a time capsule for me.  I imagine back when more than 200 HP was exciting, the Porsche 928 back then cost as much as a home and was totally beyond the current automotive design language - it still looks fresh to me.  I always get a thumbs up and smile from fellow motorists.  Today, when econoboxes have a similar power to weight ratio, I have to work harder at the throttle and shifter to keep up with traffic .... but I do.  More than 40 years later the 928 is still competitive in performance!

 

My 1979 928 was featured on the cover of the Hemmings November 2016 Motor News and also in the Hemmings August 2016 Sports & Exotic Car magzine.

Congratulations Frank, you own a piece of Porsche history.  I always recommend buying the best Porsche that you can afford, repairs and replacements are always more difficult and expensive than anticipated.