Hey brewtoo,
Just made the jump myself. Your thread echoes many of the same thoughts I had when making the decision. I have gotten to know my 300E so well I'll definitely miss her!
I went from a 1989 300E to a 1998 E430.
Is it an upgrade? Definitely yes, IMO.
The car is much more advanced all around. Initially, it felt a bit foreign to me, and somewhat 'aloof' compared to the hands-on, sturdy road feel of my 300E. However, I've already grown to love the E430 in the couple of weeks I've owned it.
The 5-speed transmission is leaps beyond the old 4-gear IMO. It shifts smoothly, no more of the solid 'clunk' of the 4 speed. The 'W/S' switch still gives you the choice of a 2nd gear start, and it is nice to have the option. The 5th gear is a godsend for highway travel. I can't comment on reliability, but my 300E has over 330,000 Kms on the original tranny. Will the 210 hold up accordingly? I hope so. For the record - although the 722.6 is touted as 'sealed for life', my tech replaced the fluid recently and recommends doing so every 50,000 Km.
Another nice feature is the ESP, which can be switched on or off. If you drive in snow as I do, it's a great feature. The option to remove it for some 'spirited' driving is nice as well. Summer roads here I come...
It's hard to compare the engines as the inline 6 to the V8 is apples to oranges. Not really a fair comparison, the V8 has so much power and torque. The grin factor it produces is substantial, however.
I had some suspension components replaced before I purchased it and it handles very well. Overall, I'd say the handling is superior. Less body roll, same tightness.
The interior is excellent and the controls are well laid out. Although the car doesn't seem much bigger from the outside, the interior space is a fair bit larger. This is something I really enjoy as I'm 6'1" and with some passengers, everyone has enough leg room!
To me, the only thing that remains to be seen is if the car ages as gracefully as my W124 did. I just sold her today and I'll miss that car. It's been the best vehicle I have ever owned - thus far. Will the 210 fill the shoes? I hope so!