For my railgun PD ships, I usually use small ships with boosted engines and minimal support systems or armor; they're there to protect the capital ships, after all, and can rely on them for refueling/long range sensors/etc.
I don't have a current game, but let's see what I can dig up from an old one.
Here's one from an older game; these days I usually call the small railgun PD ships frigates instead of destroyers.
Illustrious IV class Destroyer 7,500 tons 245 Crew 2079 BP TCS 150 TH 1875 EM 0
12500 km/s Armour 6-34 Shields 0-0 Sensors 1/1/0/0 Damage Control Rating 11 PPV 30
Maint Life 4.54 Years MSP 1906 AFR 40% IFR 0.6% 1YR 150 5YR 2247 Max Repair 937.5 MSP
Intended Deployment Time: 24 months Spare Berths 1
Large Destroyer Magnetic Fusion Drive (1) Power 1875 Fuel Use 55.11% Signature 1875 Exp 15%
Fuel Capacity 1,000,000 Litres Range 43.5 billion km (40 days at full power)
Light Railgun (10x4) Range 40,000km TS: 12500 km/s Power 3-3 RM 4 ROF 5 1 1 1 1 0 0 0 0 0 0
Railgun Fire Control II (2) Max Range: 96,000 km TS: 12500 km/s 90 79 69 58 48 38 27 17 6 0
Primary Fusion Reactor (1) Total Power Output 30 Armour 0 Exp 5%
Backup PD Sensor (1) GPS 84 Range 11.8m km MCR 1.3m km Resolution 1
And here are some other ships from the same game for comparison:
Invincible II class Missile Cruiser 18,000 tons 548 Crew 4098.5 BP TCS 360 TH 2500 EM 3600
6944 km/s Armour 6-61 Shields 120-400 Sensors 1/1/0/0 Damage Control Rating 26 PPV 72
Maint Life 5.48 Years MSP 3743 AFR 98% IFR 1.4% 1YR 209 5YR 3137 Max Repair 625 MSP
Intended Deployment Time: 36 months Spare Berths 0
Magazine 672
Large Military Magnetic Fusion Drive (2) Power 1250 Fuel Use 20% Signature 1250 Exp 10%
Fuel Capacity 1,000,000 Litres Range 50.0 billion km (83 days at full power)
Theta R400/384 Shields (30) Total Fuel Cost 480 Litres per hour (11,520 per day)
ASM Missile Tube II (16) Missile Size 6 Rate of Fire 75
Missile Fire Control FC638-R120 (2) Range 638.9m km Resolution 120
Javelin VI ASM6 (112) Speed: 50,000 km/s End: 97m Range: 291m km WH: 16 Size: 6 TH: 200/120/60
Active Search Sensor MR638-R120 (1) GPS 38880 Range 638.9m km Resolution 120
Trafalgar class Heavy Cruiser 24,000 tons 665 Crew 6657.5 BP TCS 480 TH 3750 EM 7200
7812 km/s Armour 6-74 Shields 240-400 Sensors 1/1/0/0 Damage Control Rating 35 PPV 80.64
Maint Life 5.58 Years MSP 6198 AFR 128% IFR 1.8% 1YR 336 5YR 5035 Max Repair 625 MSP
Intended Deployment Time: 36 months Spare Berths 1
Large Military Magnetic Fusion Drive (3) Power 1250 Fuel Use 20% Signature 1250 Exp 10%
Fuel Capacity 2,000,000 Litres Range 75.0 billion km (111 days at full power)
Theta R400/384 Shields (60) Total Fuel Cost 960 Litres per hour (23,040 per day)
15cm Secondary Laser Cannon (9) Range 300,000km TS: 7812 km/s Power 6-6 RM 5 ROF 5 6 6 6 6 6 5 4 3 3 3
38cm Spinal Laser Artillery (1) Range 384,000km TS: 7812 km/s Power 38-6 RM 5 ROF 35 38 38 38 38 38 31 27 23 21 19
Quad Gauss Turret (2x16) Range 30,000km TS: 25000 km/s Power 0-0 RM 3 ROF 5 1 1 1 0 0 0 0 0 0 0
Point Defense Fire Control (2) Max Range: 96,000 km TS: 25000 km/s 90 79 69 58 48 38 27 17 6 0
Long Range Fire Control (2) Max Range: 384,000 km TS: 7812 km/s 97 95 92 90 87 84 82 79 77 74
Primary Fusion Reactor (2) Total Power Output 60 Armour 0 Exp 5%
The Gauss PD on the Trafalgar gets 32 shots at a tracking speed of 25,000 km/s, compared to the destroyer getting 40 shots at half that. On the other hand, the two gauss turrets are something like 3000 tons between them compared to 1750 tons for the 10 railguns and powerplant on the destroyer, plus larger fire controls, so ton for ton they're about equal; the destroyer is 34% engine while the cruiser is 31%. And the destroyers are great for detaching to chase down and destroy missile armed ships once they've expended their ammo and begun to retreat, while also being able to fend for themselves in a beam engagement due to their speed and gross damage output. They can't really match the cruiser, sure, but that's because the cruiser also has dedicated anti-ship beam weapons.
I wouldn't really say one is better than the other; they both have their advantages. The Gauss turrets, obviously, become more cost effective the slower the ship carrying them is, and the railguns are the opposite. If you want a fast and light beam armed ship (either as an escort or a dedicated raider), I'd definitely recommend it use railguns for it's PD, though in this case the destroyers were designed as PD ships that also doubled as fast attack ships.
For the cost of one cruiser, I could have 3 destroyers, which have nearly twice the anti-missile firepower of the cruiser, nearly twice the speed, and reasonable ability in a beam fight (though they'd probably get slaughtered in a brawl with the cruiser). In the end I used a mix of both, which worked out well since it gave tactical flexibility to have a few extremely fast ships.