The thing about Slicehost, and I just had this convo on #startups, is that they can move you to metal when you get too big for your britches. Also, Linode's biggest box is 2G, which is a serious upper limit.
I know you're a mac guy, but you should look at Axure for prototyping. It's ill as all get out.
If you're targeting cheap, then Linode is a better bet - you get more for your money. There are plenty of options when you get bigger, but if you really need to save, then deal with getting big when it happens, rather than spend money on 64 bit pointers.
when you get much above 2gb, you should seriously start thinking about hardware. Especially at current market prices for a VPS, it's often cheaper to buy and host your own hardware once you get above the 2gb-4gb range. A core2duo 1u with 8GB ram? around $500-$600 for the hardware. Hosting for said 1u? as low as $50/month.
I've worked quite a bit with both LH and Tracker and I can easily say that the latter is far superior, especially in terms of the UI. It takes a bit of getting used to, but once you have your feet wet it is quite smooth.
The problem with Tracker is that it tries to be a lot more than what it really is, a ticket tracking system. Totally screwing up the do one thing, and do it well idea.
Here's my UI issues: Dense information display, MASSIVE over-use of AJAX (you hit start on a story, it just disappears to god knows where, the UI cues just aren't there), crashes Safari like crazy (probably because of all the JS in the page), every story has a billion actions you can take with it, most of them being features you will never use when interacting with the story.
Overall, I have no idea why people have been pushing Tracker, to me it is ugly and unusable. Even Trac is better. Just having lots of AJAX and rounded corners doesn't mean you have a far superior product or UI.
The only thing that made PT usable was tpope's pickler gem, and only because it is a CLI to PT.
I'd agree with everything except:
1) Basecamp -> Google Docs (I know you said this...)
2) Pivotal Tracker (honestly the worst UI I've ever seen) -> Lighthouse (or Github Issues if you're really cheap)
3) Slicehost -> Linode (but I'm probably biased and I run Rails)
4) Probably not get a CDN up front.
5) Would add Balsamiq Mockups to the list of things to get - true gamechanger