With a turbo your bike is bolted to it and held solidly in place. On rollers, your bike isn’t attached and is completely free to move as it would on the road.
Cycling on rollers: how to do it and their training benefits
Turbos allow you to create high-resistance loads and, although some rollers do have a degree variable resistance, it’s not as wide ranging. They’re also great for focussed high-wattage work, high gear/low cadence and any workout where you just want to concentrate on maintaining a specific intensity.
Rollers are brilliant for working on your balance and bike-handling skills, developing a smooth pedal stroke, leg speed and ‘rev-out’ style sessions, warm-ups/cool-downs and easy spinning.
In an ideal world, you’d have access to both as, addressing different areas of your cycling fitness and skillset, one’s not a substitute for the other. The perfect indoor workout would consist of a warm-up on rollers, main set on the turbo and then back on the rollers to cool down.
Assuming you’re getting out on the road for your long ride, from a triathlon fitness perspective, a turbo is probably your best bet. Just don’t spend all winter bolted to it though, or your bike handling will severely suffer – as will your sanity and potentially all relationships with other human beings.