Ok an update on this thread.
I went and looked at both these units and to have a chat with the retailer selling them.
Both units were of the same size and weight and both felt good in the hand.
The Lowrance looked a little more 'modern', less fussy with fewer buttons, larger screen and packed a few more features than the Horizon.
That said The Horizon with more buttons to me seemed easier to navigate being an 'old git' as I had to ask myself if I'd remember to push this button once or twice on the Lowrance to get a function I wanted

It's hard to describe the functionality of each unit so I'd recommend each individual trying these handsets out first before purchase because 'each to their own' as the saying goes
I didn't like the plastic flap covering the Lowrance DSC button as I thought it could get caught and broken off easily unlike the flush fitting rubber bung type covering the DSC button on the Horizon
After talking with the retailer they said that they had sold many Horizons over the past 3 years without many issues however the Lowrance was relatively the 'new kid on the block' and only time would tell
Happy that I could 'work' the Horizon fairly easily straight out of the box I parted with my hard earned and now that I've played with it at home I'm convinced it was the right one for me
Paul