Radial V. Diagonal

Looking at new tyres for an Explorer 1200 and find myself with a further complication to the highly subjective issue of tyre choice.

Not come across diagonals before, have they been around long? I just used to put whatever I could get hold of on my 93 Daytona as it was a little outdated on the tyre size front.

For the Tiger I'm wanting a road orientated tyre that will last and am way out of date with current types and makes. There will be plenty of two up touring and we don't want to have to start hunting round in the middle of trip for a bike dealer with the right rubber.

Is it worth going for a dearer tyre as it will be cheaper in the long run, or maybe that doesn't always hold true?

The present tyres are Michelin Road 5 on the rear and Road 6 on the front, and the back looks to be wearing alarmingly fast although grippiing like the proverbial poop to a blanket even on Irelands slippy wet tarmac.