Actually as you may know, spinal and advanced spinals are just one and two laser techs above of your current level respectively. They can always be achieved with more tech and proabably at higher rate of fire. Also you get to put only one spinal mount of the same type.
Keeping that in mind, if you plan/like to put one beam weapon to your ship that seems the best option especially if you're lacking tech wise at the begining. But, speaking of efficiency, I like to put higher rate of fire lasers and bunch of them to my ships. That being said I use spinals along with normal size higher ROF lasers on some of my destroyers and cruisers because it's so cool and RP. One of my spinals is called Melter and yeah every 30 seconds it melts.
Actually I had a good experience fiddling with a couple cruisers combat test. I really like them both, they are nearly identical except weapon sets. One of them called Radioactive, uses melter and other lasers I mentioned. The other is Cherry and wields two particle lances along with 4 particle beams, a sniper basically. Both have 600k range on FCs. Although Radioactive is deadly, it rarely has anything to shoot at because of Cherry. Cherry just erases ships.
For flexibility, I think lasers are the way to go as they can have multiple roles with high efficiency barring spinals. Because spinals feel like a bit slow and lacks the normal laser flexibility and either suitable to get the upper hand with packing a punch at low tech or they are a good looking luxury when you have enough tech.