I have licenses for both KEIL and IAR, while both are certainly good products, I prefer KEIL and use it for both ARM and 8051 variant development.
As ARM, Ltd owns or has controlling interest in KEIL, you can rest assured KEIL products have the edge on support of the latest ARM core architectures.
IAR offers compiler support for a wider range of devices, including the TI MSP430, Renesas RX and RL78, Freescale Coldfire, etc, although each toolset requires a separate license.
Of course, licensing costs are certainly to be considered as their pricing can be quite high, KEIL does offer a free limited code size version which is more than ample for many projects as does IAR and most compiler toolset vendors.
I believe KEIL has the lead as far as user base numbers, therefore when a programmer/debugger device is released on the market, it almost always comes with driver support for the KEIL uVision IDE.
BigDog