I'm in the same boat! I've been thinking about selling my TZ for TWD as well, don't know what I'll do yet but I'd be interested in what others think.

I have had similar thoughts. TZ would be the first to leave my collection. I enjoy the game but the idea of a NIB in its place does appeal. I would go with the premium TWD if I was going to pick one up, better light show and additional features. Pro is fine but if you can justify the extra $$ go premium.

I already have AMH and Thunderbirds on order both due end 2015. These are covered but my next NIB will probably be part funded from sale of TZ, RZSSI waiting to see PF layout before putting deposit down.

My plan is to have pins from different manufacturers with no more than two from any one. I will not pay over 10K for a pin so Woz RR(13K) and TWD Prem(10+K) not interested.
2 Bally's. TZ / DrWho
2 Stern's. TSPP / Met Monsters
1 Williams. BSD
1 Capcom Pinball Magic
1 Gottlieb. Wipeout

TWD will keep you coming back. Especially with the updated code.

Having only 4 games, I think switching games in and out is a good idea. TWD is an excellent choice IMO, I recommend the Premium because I think you get a lot of nice upgrades: WW Magnet, Crossbow, BG Ramp/Bash toy, Spinners, RGB inserts, color changing GI, Lockdown bar fire button (Walker Bombs). I also recommend a shaker motor for TWD as it is integrated well.