If someone is inocent then doesn't that pretty much lend itself to the idea that there isn't much hard evidence to go on?
No, not really. Evidence which seems compelling enough for an arrest can later turn out to be false, misinterpreted, countermanded or insignificant. But that's why an arrest doesn't instantly lead on to sentencing: you have a trial in between.
I guess what I am asking two questions in this thread. How much evidence do you need to create 'reasonable suspicion"?
Legally, the answer is roundabout: evidence which would lead a reasonable person to suspect they may be guilty of the offence.
In reality, that means something that ties them to it, a witness, their presence where they shouldn't be at a crime scene, something like that. For example, if you come to a crime scene and Mr X is standign over a body with a knife and blood on his hands, I think a reasonable person would agree there is likely to be grounds for arrest on suspicion of having done it. Mr X may
not have done it, and he'll have the chance to argue that later. Perhaps he performed CPR, getting blood on his hands, and then pulled out the knife. It would be up to the prosecution to prove that was not the case. But at the time, there was reasonable suspicion that he was responsible.
The second question is this. What is a bigger sin. That a incocent man is arrested and jailed or that a guilty man escapes justice?
Depends what you mean by 'jail'. If you mean arrested and held for the legally prescribed time then charged and bailed/remanded or released withotu charge, I don't have an issue. If you mean convicted and sentenced for the crime, yes that is worse than the guilty person getting away.
I think it depends on the nature of the crime. It's better to inconvence someone to stop a serial killer than it is to try and find out who stole your tv set.

You don't consider theft to be an offence worth arresting someone for? I think you might feel differently if it was your stuff.