With inflationary currencies created from debt, I get stuff now, but then I have to pay more for it later due to interest and inflation.
With Bitcoin, I pay for bitcoins now, then I get stuff later by either paying less due to appreciation of value of the limited-supply currency (if the adoption keeps growing), or pay more due to depriciation in case people start dropping out for some reason, and value of coins dropping as a result. Both scenarios happened in the past at some scale.
I think you are mixing two independent things and it confuses you into wrong conclusions.
One thing is upfront payment vs. post payment. If you pay after you got stuff, you just have a debt to the seller until you pay, so you just need to pay more as compensation for that loan and for the risk that you might won't pay. If you are willing to pay long before you get your stuff, you might pay less, because it's you who gets compensated for a loan and it's you who risks that they won't deliver. Both these situations are possible in USD and in BTC. Just take a look at Kickstarter as an example.
The other thing you mixes is depreciation or appreciation of currency value. This is a variable regardless of the upfront/postpay sale model. E.g. if you agree to pay 5% more one month later and then the value of currency drops 10%, you might end profiting 5% even despite you pay later (because that 105, which you agreed to pay instead of 100, is less than 110 which the stuff costs now after currency value dropped 10%).
In case of Bitcoin, it's a level field and open book for everyone, but we don't really know how much our savings and contracts will be worth in terms of goods and services they can provide.
You never know how much your savings and contracts will be worth in terms of goods and services they can provide. Period. Regardless of currency. There are more stable and less stable currencies. USD is more stable than BTC, but then you have the Zimbabwean dollar. Appreciation/depreciation risks are not exclusive to Bitcoin.
Of course, inflationary systems mean in general that your savings are expected to be worth less in the future and deflationary systems mean in general that your savings are expected to be worth more in the future. The thing is, there are these things like demand and supply. When you talk inflationary/deflationary you consider only supply, but it's the demand that is so much more important. If people stop using BTC, it will lose its value despite being “deflationary” in general. On the other hand, take a look at this EURCHF 5-year chart:
http://finance.yahoo.com/echarts?s=CHFEUR%3DX+Interactive#symbol=;range=5y - despite fiat currencies being “inflationary” in general, CHF appreciated 33% between 2008 and 2011 because of increased demand (it's considered very safe so it has increased demand in times of crisis as investors tend to move some part of their capital into lower risk instruments).
So these things you mention are two completely different issues and they apply to all currencies, not only to bitcoin. You can have upfront payment in inflationary currency and pay less (Kickstarter). You can have value appreciation in inflationary currency (ex. CHF in the last five years) and value depreciation in deflationary currency (gold prices between 1980-2000). Just don't think there any hard rules here.