I know your car quite well I think - black 135i, Quaife diff, Dinan intercooler, Hartge suspension and rollbars...??
You've seemingly gone about improving your car the same way as I did - make sure everything works within tolerances, handling is spot on etc etc before upping the power. Now that you're considering the power, you actually have four options - Birds/Dinan, DMS, Evolve, and BMW.
I think we can discount the last option - BMW - as it really isn't a valued upgrade. All the other remaps, whether the Dinan from Birds, the DMS, or the Evolve, are all much of a muchness. Yes, there are degrees of differences between them, but to be honest the power output is pretty similar. Driveability, or rather the way the car drives, will be a little different, as each tuner has a different philosophy on how to best extract power and torque out of an engine.
When I remapped my car, I went with DMS largely because I have a fairly long history with them and I had very specific requirements for what I wanted to do which I know DMS could fulfill. That is in no way disrespectful to Evolve - at the time that was the best option for me. Kevin had just started with the Dinan software, but it was new to the UK at this stage and it required the ECU sending over to the USA for programming. it was a fairly laborious process. And expensive too!
However, there is the very valid issue of warranty - the Dinan tune comes with a US/Canada warranty, that theoretically is extended to the UK as well given there is now an official supply deal. The main issue with the Dinan tune has always been one of cost - compared with Evolve and DMS, it was much more expensive. Obviously the cost includes the benefits of the warranty, and that can't be overlooked. However the cost was high also due to the fact that ECUs had to be sent across to Dinan in the States, then sent back, with customs duty on top!!
I do know however that Dinan are very soon coming to the UK with their new programming tool, so that Birds will be able to program the Dinan tune onto ECUs directly from their workshop. The price of the Dinan tune in the UK is set to come down quite considerably, and this will put a very different spin on things.
In terms of reliability, my car is pushing out a monstrous amount of power and torque, and it is doing it completely without fault. I have made ongoing changes, for instance using catless downpipes, and the remap has always adapted to suit. Evolve, Birds and DMS all offer different states of tune, from 360hp upwards to about 390hp. You have to decide how far you want to take your car, and also you must bear in mind what sort of use you're going to put your car through.
One major word of caution - if you are planning on tracking your car extensively on the more demanding circuits (not little ones like Brands or Thruxton, but like Spa or the 'Ring) then you will find very quickly that your car will overheat due to the limited capability of the standard oil cooler that BMW specify. This was apparent on my car even when the mapping was standard - the oil temps reached almost 140 degrees C - limp mode is achieved when oil reaches 147 degrees - it's far too hot!! If you map the car to 380 let's say, you really will see oil temps rising, despite the fitment of the intercooler. What you will need is to upgrade the oil cooler - Kevin has multiple options available to you, from the Dinan being the most expensive to other alternatives a fair bit cheaper. I use one from the States, not Dinan, and it is supremely effective - oil temps regardless of how hard I push never go above 130 degrees max - normal driving sees it stay around 105 or so.
So, at the end of all this, what do I think you should do?? Well, given you've already gone down the Dinan route, it makes sense for you to stay with the brand and get a Dinan tune. You know it will work with the intercooler, there's the warranty aspect, and the cost is going to come down once the programming tool arrives in the UK. I know the approximate schedule, but it is still to be confirmed so I won't mention it for now. I suggest you get back to Kevin and discuss it further.
And if you do decide to go with Evolve or DMS you won't go wrong either