I thought I would chime in here as I own a ti, a Z3, an E46, and an E21... I have owned several others over the years and I am pretty familiar with the suspension set ups in these cars.
The E21/E30 and Z3/ti all share the same basic design. Jess has made the point that many (all) of the parts for the ti/z3 rear suspension are not exactly the same as the e30 but the reality is that they are not that different... the differences required slight tweaks, not a complete suspension redesign and retool.
From a driver's (or my) perspective what the E30 style rear suspension gets you is a lot of fun. A 325ti can be a bit of a handful and it is a car that demands your attention to drive fast well... an E36 325i can be driven fast less dramatically and also feels much more stable and composed at high speed. Some might find the E36 325i less connected... more generic... to me it is just the nature of the 2 different suspension set ups... and the E36 multi link set up is just as 'disconnected' as the E30 set up is 'old and antiquated'. There really is no right or wrong, each set up offers 2 different feels and whatever feels the best to you is the best for you.
Sub-frame failure... I am very familiar with this as I just repaired the cracked floor in my E46. I am not sure why it is called 'sub-frame failure' as it is never the sub frame itself that fails but the floor above it. Standard E36's had the problem, Z3's had the problem, and BMW settled a class action suit for the E46 models with the problem. The problem is not limited to multi-link or semi trailing arm suspensions but it is fortunate that the ti seemed to evade this issue. Having just totally rebuilt the rear suspension in my E46 I feel it is a much more tuneable set up than that on the E30/ti. On the multi-link rear suspension, all you need to add is a set of camber adjusting arms and you have control over everything you need back there. On the ti you have to do some welding (or trust offset bushings to stay put) to gain control over rear suspension geometry. While the multi link set up is more complex and has more parts I don't feel like there is that much of a weight difference. The sub frame itself is small and more like a cage around the differential, not something that goes from one side of the car to the other. The arms the springs sit on are aluminum, and the trailing arms are hollow steel.
E46 multi link:
Owning a Z3 Coupe for the past 10 years I can confirm that there is a lot about that car that was not well thought out and seems very much slapped together... The moonroof has no actual cover so you just sit in there and bake in the sun... The fan for the climate control system cannot actually be turned off, it still blows even set at 0... M cars have no spare, but they do have an extra (and pointless) muffler as well as an (also nearly pointless) m-Mobility Kit (BMWs version of fix-a-flat and an air pump)... The battery in the MZ3s was in the center of the trunk because they had nowhere else to put it... in all the other Z3's but the 1.9 the battery is in the right rear of the trunk. Z3 windows do not go all the way down... So to think that BMW's reason for using the E30/ti rear suspension was to save money is not that much of a stretch for me. That said, the car is great fun and at this point I consider it an old friend that I would not change at all.
After all that there seems to be only one real way to settle this... an equally equipped ti, vs and equally equipped standard E36... but in the end it will prove nothing, drive what you like and don't worry about the rest!