How long is a piece of string? Seriously this is such a personal question. Some of the detectors I would like to use are so heavy that you would need to be in the prime and work out regularly to use them!
Those that give the sharpest signals on small (very small) targets have little or no depth penetration.
If you go to rallies up down the country you soon notice that certain machines are more popular in one area than another, now the manufacturers all claim their machine is a go anywhere machine but I have my doubts.
I once recommended my favourite machine to a couple of friends who lived in another county, sadly they took my advice and had no luck at all. Thinking they didn't know how to use the machine I went out with them and found I had the same problems, my trusty old friend was just not suited to their soil conditions.
It all depends where you want to search.
I have a Whites XLT metal detector and it is reasonably good because it is very adaptable. However, I have found that in certain ground conditions cheaper detectors can give better performance than my trustworthy XLT.
With regards to the Whites XLT, in the usual ground conditions round here, it does a reasonable job at IDing things correctly when they are less than four or five inches deep. Any deeper and it will just ID them as ferrous or non ferrous (with approx. 70% probability of being correct).
I personally like the combined all metal/discrimination mode of this machine, which I use if there is little trash or iron contamination. It will detect deeper than many machines, but remembering that it will not ID the deep targets.
The Laser Rapier, as well as being really easy to use, is also very lightweight – it only uses a single transistor battery – OK it won't last as long but it's easy to carry a spare battery in your pocket.
If it had been available when I first started, I would almost certainly have gone for it. Otherwise, if you go for a heavier machine, you may find that one which allows you to belt-mount it is easier on the arm. The only problem with belt mount is that the unit can get in your way when you bend down to dig, also it can be less easy to read if it has a meter or display. I now chest-mount my Laser B1 Hi Power using an excellent harness made specially for the purpose which came from Joan Allen. This may or may not be suitable for you.
The Lasers are good machines, although you have to be a bit patient for the first few months in learning to "read" the signal, since they're sound only. However, I believe that the simpler the machine, the more time you'll spend searching and the less time fiddling about with the adjustments. The lasers are excellent at discriminating out small iron – nails, etc, however the signal from large, deep iron – horseshoes, ploughshares etc can sound like a good signal.
The only way to learns is to dig every signal at first until your ear is tuned in. The XLT will potentially give you a lot more info, if you're prepared to spend time fiddling with different programs.
If you haven't already done so, make sure you go to dealer and try several machines out for size before finally deciding.
Another thing, don't forget the accessories.
If you're going to spend that much on a detector you may as well get some decent headphones too, I just bought a pair for under $100 and the difference is unbelievable. They practically stop any outside noises and all you have to concentrate on is the signals coming from your detector.
Definitely worth it, so make sure you put enough aside for the headphones too, you may already have a pair but I was using the ones I use for the CD system, these new ones are amazingly clearer.
At the end of the day you cant beat going to one of the detector shops, trying all the machines until you find one that you think does the job you want, has the right sound and a good recovery speed after it's passed over a piece of iron (That's the time before it will pick up a coin after going over the iron) and suits your height and weight.
Then once you have bought it, get to know everything about, don't just use it, learn all it's in and outs, sounds and meter response. After that just remember you cant find what isn't there to start with!