There's different kinds of reliablity really. In my opinion, it can be broadly divided into:

Hardware
Software
Support

I've owned many phones over the years and had the experience of seeing general reliability of many more through family, friends, etc.

My personal history of phones goes a bit like this:

SE T68i
SE W800i
SE K800i
Nokia N73
SE K850i
Nokia N82

I would have to say that in terms of general solidness and build quality, the SE models (that I've used) wins out. However, there were teething problems, and I think it's something that's plagued SE for a long time and it was the joystick directional control not being very reliable (due to dust, etc). My W800i also managed to get dust under the housing screen somehow too.

In terms of software, the SE's have been almost static for a long time now. I don't mean this in a bad way, in some ways it's good because it's a seamless transition in going to other SE models. However, I find the OS very restrictive. Only really java applications can be installed unless you do some serious hacking. The software due to it's closed nature does mean that it's very stable though.

With the Nokia's, yes the first generation of Symbian (especially v9) was slow, but it made up for it with it's ability to be extended and customised with 3rd party applications. This can come at a price of stability, but in general it's very good. With the latest Nokia's the OS has speeded up considerably and is on par if not faster than SE OS.

Unfortunately, I can't comment on support since I've never had to deal with either of them.

In recent times though, I think that SE has fallen behind Nokia. My current phone is the Nokia N82 and it's build quality is extremely good as well as having an awesome feature set (wifi, gps, 5mp camera + xenon flash). I can't really see anything too innovative about SE phones since the K750i.