I train scientists in how to operate and service many brands of HPLC systems. I think that the HP/Agilent systems are some of the easiest to learn and most reliable HPLC’s available.
*Do not worry about sensitivity issues. Sensitivity has more to do with the quality of your method and column choice, how well the system is plumbed, your choice of flow cell and most importantly the detector settings than which detector you choose.
The HP DR-5 powered 1090 was the Rolls Royce of its day. We still use them in our lab and love them (20 years old and they run perfectly). A two hundred fifty plus pound monster, but capable of amazing things. However, they can be a nightmare to service compared to current systems. *If you are not a highly technically skilled person and naturally mechanically inclined, then I do NOT recommend an HP 1090 for you. Wonderful machines, but they were really designed to be used and serviced by the minority of users. *Can you tell I like them? But I think I was born with a wrench in one hand and a multimeter in the other.
The HP 1050 and HP/Agilent 1100 systems are 95% Identical since the 1100 and 1200 series were all cosmetic re-works of the HP 1050 system. The HP 1050 introduced the still current pumping system design which is virtually unchanged to this day (thank goodness they did not mess with it). It has variable stroke volume, adjustable compressibility, extreme reliability because of the computer controlled active inlet valve and does not require syringe priming. Besides repackaging, basically the only changes made to the 1100/1200-series were: the separate keypads were deleted; the Autosampler was upgraded to use electrically actuated motors instead of pneumatics; the capacity of the std A/S was increased to hold 100 vials and the diode array detectors benefited from some minor technology upgrades. These are really EASY to service and operate.
Buy the system you feel most comfortable with. If money is of concern then consider either an HP 1050 system or for a bit more money a used HP 1100-series HPLC. Most of the parts are identical between the two systems (Note: Different A/S so some of those are different). Get a diode array detector (DAD) equipped system if at all possible. You will not regret it and the extra information provided by the DAD will save you time and money in the future developing sound methods.