I am aware of the differences between the XP-30 and XP-60 - ie. no sequencer and three sound boards on the XP-30.
What I was wondering is, does the XP-30 sound better than the XP-60. I've heard the famous decay problem on the XP-60 and I was wondering if it can be heard on the XP-30, I personally couldn't hear it on the 30.
I would ideally be looking for a sequencer as well, so the '60 sounds ideal but I'm concerned that I'm going to regret it with poor sound quality. Does the expansion boards have the decay problems as well? Also the XP-30 would probably end up costing me more as I would have to buy a hardware sequencer.
Any advice would be welcome.

Any reason you want a hardware sequencer rather than a software sequencer (no PC at home?)?

The XP-30 has the same "decay noise" as the 60. The "decay noise" is present on all sounds, including expansion board sounds, as it is an audio phenomenon (likely a AD/DA conversion issue) and not something to do with the waveforms or synthesis.

I love having an on-board sequencer. I can tote my 60 anywhere and sequence, program, practice, or perform (including using RPS which the 30 can't do). If you do plan to purchase a hardware sequencer, I'd say just buy the 60. There are a lot of rarely-used 60s for sale on eBay from time to time. Just make sure you ask the owner to tell you what ROM/CPU his or her 60 has installed (enter SEQUENCER mode, then press SHIFT + ENTER + EXIT simultaneously) - only buy a synth with ROM 1.10 and CPU 1.02 or later.

Good luck!