There is, with 10-holes, the very important issue of TUNING, which I'll come back to.

I don't know enough to be able to give a rundown on all the available 10-holes. I don't pay more than about twenty quid for any of my 10-holes (or at least I didn't until the XB40 came along...see below), and I get what I consider to be excellent instruments for that price. You can pay more for harps with various refinements, such as valved reeds to facilitate note-bending, but, with the exception of my two XB40s, I haven't strayed into such territory as yet. You can even have your harmonicas customised to your requirements, but before going to this expense I think it's worth getting to grips a little with how your harps work - see the next page, on set-up and tuning - so that you'll know exactly what you want your customiser to do for you. The only other thing I would say on choosing harps is that I've found that really cheap harmonicas are mostly false economy.

Apart from 10-hole diatonics I also play double-reeded instruments - the ones that give a vibrato or "tremolo" effect. I like the additional variety to my "sound" that these give.

WHICH LEADS ME to enlarge upon the subject of my favourite harmonicas. As to the matter of WHICH KEYS, if I were to roll up to the session with just three 10-holes, in D,G and A, I would have a happy evening. C can be useful occasionally too. My G and A harps are Lee Oskars or the very similar Tombo Folk Blues (Lee Oskars are made by Tombo and have the same plates). Lee Oskars are well-made, with the accent on quality control, quite loud and VERY durable (easily the longest-lasting reeds I've come across). They also have the advantage of having replaceable reed-plates - a useful feature shared with those Hohner 10-holes with the letters "MS" (Modular System) stamped on their covers. These Hohners,which exist in several manifestations such as "Blues Harp," Cross-harp" and "Big River," are similar in price to Lee Oskars (Big Rivers are cheaper). Some have plastic combs and some have wooden ones (personally I can't detect much, if any, difference in tone, though I find those with plastic combs much easier to clean) but the reedplates are interchangeable. I have some MS blues harps (wooden combs) and I like them, but the reeds don't last as long as Lee Oskars.
The key of D is a bit tricky. It is the most frequently-used key in Irish music. The problem is that the standard D 10-hole harp is high in pitch and will often put you an octave above the fiddle player. The solution is to get a harp pitched an octave lower: a low D. Lee Oskar don't do them. Hohner MS harps in low D are available but I have had problems with these. The one I go for, and my very favourite harmonica of all, is the Hohner Special 20 low D hand-made harp. Replaceable reed-plates are not normally available (though it may be worth asking specialist retailers), and the harps haven't always been easy to get (though the situation has greatly improved in recent years), but for me they are the answer.
As for my tremolo instruments, I've settled for Hohner Echos. The double-sided ones are very useful,as the two sides are in different keys. I have two of these: one in D and G and one in D and A. Mine are 96-hole models and cost about £60. The pitch of the D scale isn't a problem with these - but see my remarks on tuning a little further on.

My latest acquisition is the Hohner XB40. It's a 10-hole harp with a difference. Every "sounding" reed has its own auxiliary reed (hence the "40" in the name), a feature that enables every note to be bent easily. This revolutionary harp extends as never before the potential for added expression and more "chromatic" playing, though you do have to have thoroughly sound mastery of technique to get the best from it. So far, I have rather mixed feelings about its tone and responsiveness, but it is a very different beast to the standard 10-hole and I may well find that it "breaks me in" eventually and I'll become a lot happier with it. I will of course update my views as time goes on. They are much louder that any other harps I've ever played, which could of course be an advantage in a noisy setting, but I do find that I have to play them very hard to get a good response. At the moment my XB40s tend to remain in their pouches. The XB40 will set you back more than twice as much as a decent quality traditional 10-hole would, so it's quite an investment. They are available in many keys now, including low D, though I could only obtain the latter from the USA. You may have better luck.