I can't disagree with anything that's been said, but as I've ridden all the tyres mentioned I thought I'd give my tuppence worth.
Any tyre choice is a compromise between performance, convenience, and cost. Where you want to be on that curve is down to personal choice and what kind of cycling you do.
Performance includes weight, rolling resistance, and grip. The last two tend to trade off against each other. Convenience includes durability (how long they last), reliability (i.e. puncture proofness), and maintenance (e.g. how easy to get on and off). Wire bead is heavier, but easier to mount. Folding tyres have a kevlar bead which is lighter, but doesn't stretch over the rim so easily.
My view on cost is, pay for the right tyres to do the job you need them to do. How much would you pay not to be repairing a puncture by the roadside on a cold, wet, ride home at night? Add a few of those up and you quickly justify the investment.
Flak Jacket isn't Specialized's top puncture proofing. That would be Armadillo (bead to bead Kevlar), also available on the Nimbus. But the tyres you have are probably a good compromise spec for the type of bike you have. For the record, I once rode a Nimbus Armadillo to death without a single puncture for several thousand miles until it ran out of tread. I've seen a thorn embedded up to its armpits without puncturing the inner tube. On the down side, they are a bit weighty; and once I had an issue with a tread delaminating. Also, because Specialized are imported there is a price penalty; and they can be slightly harder to source.
Schwalbe, who make Marathons are a German company (thanks for correction
@andrew_s). I run Marathon Plus on a couple of bikes - heavy touring/winter commuting types. Never had any issues with them. Reliable, and good grip. They are the gold standard commuter tyre for anyone not concerned with 'performance', as attested by the many posters in this thread.
If you want something a bit lighter and with less rolling resistance, that's where Continental (German) score. Their GP4000 SII are considered by many to be the benchmark road tyre, with low rolling resistance AND good grip - but probably a bit vulnerable for regular commuting or any off-road. Their most reliable offering are Gatorskins, which are great for summer commuting, but the slik tread is a bit iffy for wet roads or muddy tracks. They're also a bit weighty and ride like rocks. The compromise is the GP Four Season mentioned by
@Fonze, which hits the sweet spot of low rolling resistance and weight, good puncture resistance, and good grip in most conditions.
Ok that was a tanner's worth after all.