The Pinnacle is better value for money. It generally has higher specification components (e.g. Shimano's Sora range is better quality than its Altus and Tourney ranges used on the Trek), and carbon forks with an aluminium steerer as opposed to an all aluminium fork. That is not surprising since Pinnacle is Evans' own in-house brand and you will inevitably be paying a bit extra for the Trek brand name (and I suspect that the Pinnacle offerings at this price point are at least as good if not even superior to their Trek equivalents).
Whilst I like triple chainsets, in this case I would prefer 2 x 9 speed Sora components over a lower quality 3 x 8 speed set up. A visual representation and comparison of the two bikes' gear ranges is
here - as it shows you get a lower bottom gear with the Trek, but I would not be concerned about that unless you were riding in very hilly areas or carrying camping kit. The top gears are pretty much irrelevant: it's unlikely you would ever need to use the 11 tooth sprocket unless you were sprinting downhill.
The Pinnacle comes with 28mm tyres, which is a reasonable size for riding on reasonable quality roads (and probably ideal for your triathlon), but you would probably find wider tyres more comfortable for poor road surfaces and especially for towpaths, bridleways and gravel tracks. According to Evans' website the Pinnancle has clearances for up to 40mm tyres, but it does not say if that is with mudguards (so I suspect with mudguards there might be clearance for 35mm tyres). Both 35mm and 40mm would be better for light off road riding than 28mm. If I were buying the Pinnacle, I would ask Evans if they would fit mudguards as part of the deal if I bought the mudguards at the same time, and I would ask them to fit the widest mudguards the frame would take (i.e. I would not want narrow mudguards which were only suitable for the 28mm tyres the bike comes with; I would want mudguards suitable for at least a 35mm tyre, even if I kept the 28mm stock tyres on the bike to begin with/until they wore out).
However, the most important thing is that whatever bike you choose fits you and feels right/comfortable. It's like buying shoes: it doesn't matter how low the price is, how good the quality is or how good they look, if they don't fit and hurt your feet.
