Welcome to the forum!
I'm a happy MS1i owner, and I've tried a number of other Grados over the years: 225i, 325is, RS2, RS1i.
First of all, if you are used to the sound of the MS1i with L-cushions, don't expect a big change from the sound of any of the the Grados you are considering. There will be differences, but they are quite subtle and you have to listen out for them. I was quite disappointed when I first tried the higher end Grados, as I'd read a lot of comparisons and was expecting to be 'blown away' by the differences ... I wasn't. I eventually traded up to the RS1i and, whilst I really liked them, I didn't think the difference was quite worth the money at the time (I need the money for something else, in other circumstances, I'd have kept them). I'd always kept the MS1i as my 'reserve set' and now I'm just left with them. In the end, I realised this confirmed just how good the MS1i is, especially for the money!
Secondly, the 325i / MS2i are actually quite a bit heavier than the MS1i - not just because of the metal cups, but also the cable, which is a lot thicker and tends to pull the headphones down a bit. It's not a problem for many people, but I personally found them too heavy for long-term listening. I'd personally say that the RS series are the most comfortable Grados I've tried. Ultra-light and they just seem to 'sit' better on my head.
I'm not trying to dissuade you from upgrading by the way, just offering an alternative viewpoint! For what it's worth, if I was in the market for a new Grado and wasn't on a budget I'd probably go for the 225e or the RS2e ..