dealing solely with 1640 vs. 1620i response, as always is 'depends'
do you need (now or in foreseeable future) more then 8 pre amps?
if more then 8 then 1640, even as a discontinued model suggests itself
The 1640 has four sub groups, plus a mono out (in addition to L/R mains) (both missing on 1620i)
1640 also has direct analog outs (even with firewire and channel inserts direct analog outs can be useful)
If you are using it primarily for in home recording and don't see the possibility of needing 9 mics on a drum kit there is probably an argument for going with the new product
If you are planning on using it for live sound reinforcement for any but the most basic set ups I'd find the 1620i to be a bit limited . . . can be used but either you'll spend a lot of time cobbling work arounds or settle into a single intransigent set up
While I don't perceive any killer upgrades, more feature creep with some useful items missing, on the 1620i . . . onyx mic pres are certainly usable but nothing I've seen suggests that they suddenly crossed into a new level with new product series. If you are using it primarily for home recording any slight improvement in channel EQ should be only marginally significant.
without doing head to head comparison (old EQ to new) I can't be sure but would be surprised if, for typical live use, the difference were make or break. The Channel strip EQ on the onyx series was functional (at least marginally better then several other entry level brand boards) but fact is for anything but most rudimentary set ups you still need outboard eq . . .
in your situation I would probably be torn as well . . . the newer model that might have sufficient feature set for near future (or forever for that matter) or discontinued model with significantly more I/O options?
At the moment I'm currently using an Onxy 12xx series for a couple of live acoustic duos and trios . . . with more flexible output (then 1620i) (& fewer mic pre's) even the small acoustic ensembles strain the abilities (but it's light weight and my next 'up' option is a 2408 A&H) . . . at some point in the next year I'll probably upgrade hardware for this niche and onyx 12xx board will go back to being a 'returns' sidecar in the project studio
good luck