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Generally, the tri shoe will have more mesh in it as the triathlete transfers from the water to the bike and wet feet go into the shoe. Also, tirathletes typically pull their feet out of the shoe and ride on top for the last few feet before going into the transition and the third dicipline, the run, so the softer shoe helps there, as well. They also typically have one velcro strap to speed up getting on and off the bike. In most of the country, in colder temps, all that fabric and mesh is not good enough for regular riding. I have both as I use road shoes to train in then my Tri shoes to race in. If given a choice of one only though I'd go for the road shoe. Better upper structure for your foot for regular riding.
and don't buy them too small. Don't know if it's just my feet or women's shoes in particular, but I've found I have to buy shoes two sizes bigger otherwise they pinch my feet and cause terrible pain.
My top tip is to buy road shoes with velcro straps instead of loads of buckles. They're much quicker to get on and off in transition.
FB_Princess, it's well worth investing in good quality road shoes.
Basically what I'm trying to say is make sure you try before you buy (no use ordering online if you've never used them before). Go to a bike shop and get a proper fit where you can try various sizes, male/female ones (male shoes are slightly broader) and some offer soles that get heated moulded to fit snugly for your feet.
Don't forget to consider which pedals you'll be going for as some shoes can only use recessed cleats such as spd or non-recessed ones such as the Look systems. Ask before you buy!
My first shoes were ladies shimano basic road shoes, they were my size but way too small and horrifically uncomfortable. I also didn't like the velcro straps (sorry Mr. W!) My second pair were again ladies specific but this time Specialised road racing shoes, also size 6 but this time I removed the inner sole which made it more comfortable. However after a few hours riding I'd still get numb feet. I've now bought a men's Specialised shoe size 6, carbon sole so no flex, ratchet system with part mesh material. I had it heat moulded to my feet but didn't like it so have now removed the inner sole again. So far so good.