It all depends on what you want to actually do, they are two very different pieces of kit.
A CNC tends to be a router type machine that either engraves the likes of glass and steel with the right type of bit and then also works as a router type machine.
The engraver machine is just that, it engraves only, there are several different types out there, CO2 or the light laser (similar to the laser pointer only much more powerful)
I know [MENTION=284]johnny5[/MENTION] has a CNC he might be able to give you some thoughts on that, and if I remember rightly [MENTION=1682]KevinMc[/MENTION] has the light laser. "Neja" type machine.
I have a 40w CO2 one, which is a very good machine now that I have "customized" it to what I wanted to do. The pain to get it where I wanted hasn't been an easy one, however a friend of mine has the 100w unit that he works straight out of the box. no issues.
It all comes down to what you want to pay, Chinese and Taiwan imports are common and a very cheap option, however, they are cheap units so be prepared for some heartache with them.
You can get a NEJE light units for less than £100, CO2 units start around £300 and a small CNC you can get for around £400.
Bear in mind with them most nearly always use G Code to operate and you will need to get your head around that and the software you decide to use. You will also want to decide if you would like a rotary axis as well.
There are forums and G+ communitites for Laser engraver and CNC so if you want to go that way do your research first.
There are also forums that cover both machines such as CNC zone.
K40 laser cutter upgraded to AWC708c controls
MYCNCUK
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But.... If you want anything particular engraving then PM me and can always look at what we can do.
PM me your tel number if you want to talk it over as sometimes its difficult getting all the info you need over here.