The purchase decision is difficult as there is no way of telling when Keysight will release the successor for 2000X and 3000T X-Series oscilloscopes. Trying to decide if I should just get the GDS-2204E (200MHz, 4-channel) now or just go ahead and get the Keysight 3000T X-Series. The 2000X is a bit outdated as you all know. The GDS-2204E is USD1570 after 20% off discount from GWInstek. Local distributor said that they cannot offer more discount than that. From the "New Rigol DS1054Z oscilloscope" thread, there seems to be some issues. Perhaps it is still better to get the GDS-2204E even it costs 1K more?
I heard that for Keysight, there is no upgrade from 2 channels to 4 channels. So, perhaps 100MHz, 4 channels is the one to choose if I end up choosing Keysight?
Honestly, you're going about this the completely wrong way. The fact that you consider a $400 (DS1054z) together with a $3k+ scope (DSOX3000T) shows that you don't really know what you actually need. And without sorting that out first, you've set yourself up to a path that will very likely end you wasting a lot of money.
So the first step is to decide what it is that you want to do with the scope, now and in the foreseeable (say 3-5yrs) future.
1. What stuff will you work on with that scope? What type of signals do you want to assess? => this decides the frequency range which in turn tells you what scope BW you need (and be careful with the waveform type, i.e. a 200MHz scope can't show a 150MHz square wave correctly).
2. Can you do your work with standard passive probes, or will you need active probes? If you'll need active probes then you need a scope with active probe ports
3. Do you need logic analysis? If so, is a scope built-in SA (MSO) good enough, or is a separate USB LA (like a Salae) a better choice?
4. Do you need serial decode? If so for what protocols? => this might remove some scopes from the list as not all scopes support all protocols. Also consider an alternative to scope serial decoding, i.e. via a USB LA.
5. Do you need FFT? If so then you want a scope with strong FFT performance over a large sample size (i.e. 1Mpts)
6. Do you need specific scope applications (i.e. power analysis? Report generation?)
7. What's your max budget?
You should really step back and try to think about these seven points so that you can answer them with a certain confidence. Don't sway off into wishful thinking, try to stay realistic with what you want to do.
Once you've sorted that out you can then start narrowing down which scopes fullfill your requirements. And for that it's irrelevant if a manufacturer comes up with a new scope model next year, because it won't make the scope you bought less usable for your work. And that's all that matters.