Counting the Cost - 15mm vs 28mm Sci Fi

I have been looking at the lovely new 20mm/1:76 for Elfheim and pondering the pros and cons of scale.
The 20mm has benefits of being able to use cheap and available 1:76 vehicle kits. However I am not in the least interested in making AND storing another bunch of terrain. 

So I will look at 15mm vs 28mm, with an eye to modern and sci fi forces.

TERRAIN: STORAGE vs DETAIL vs COST
15mm wins hands down.  The commercial buildings are rather good, cheap (at $5-15 per building) and you can fit an entire resin city in a 13l box.  28mm resin buildings can be unviably expensive, ranging from $30-60 for  the average building. If you scratch-build your own, it is more even - 28mm is more forgiving if your measurments and cutting are not up to scratch.

Most 15mm games use a 4 x 4 board. A card table vs the dining table is handy for those pressed for space.
Finally, 15mm looks better 'in-scale' on the table.

MINIATURES: DETAIL vs COST
For skirmish games (10-20 individually based minis per side) I favour 28mm. For platoon sized games (half a dozen squads, plus vehicles - like a 40k game) then 15mm is superior.

With vehicles, there is little difference in finish and quality.  Vehicle heavy combat is where 15mm shines.  But despite the massive gains in 15mm sculpting, with the infantry, size DOES matter - 15mm will never match 28mm in sophistication (in sculpting and the ability to apply a quality paint job). So for small skirmishes not involving vehicles, 28mm is a clear winner.

Some 15mm can masquerade as 28mm though:

Miniatures by Khurasan

Cost.  For a benchmark I am using a 4-APC platoon, with 8 x 4-man infantry fireteams, plus some support fireteams and a scout vehicle.  I am avoiding GW products due to their pricing structure.

28mm Infantry (Pig Iron)
#Models   Type    Cost
20 x Colony Militia  $50
8 x Specialists $26
4 x Missile team $13
4 x Sniper team $13
Total $102

28mm Vehicles (Antenociti's Workshop)
#Models   Type    Cost
 4x Hunchback APC $120
1 x Zebu 4WD  $20
Total $140

15mm Infantry (GZG)
#Models   Type  Cost
24 x Infantry  $15
16 x Support $10
8 x Missile/sniper team $5
Total $30

15mm Vehicles (GZG)
#Models   Type  Cost
 4 x APC 8 wheeled  $40
1 x Scout car $10
Total $50

Cost (ignoring 15mm's superior P&P) is $80 vs $240.  To put it in a different perspective, a complete 15mm platoon with 5 vehicles and 40+ troops costs the same as a single GW tank.