I have one of the USB 'scopes similar to your 2nd link. It's noisier than a quality scope.
Just curious: is the noise typical random 1/f noise or mains/hum-related? IOW does it help to use it with a laptop that is under battery power?
As an overall comment, strictly for garage use I'd probably first look at hand-held, battery-powered LCD display scopes just because cables (USB and/or power) are pretty inconvenient around cars, as are laptops and normal-sized scopes since you have to put them somewhere and a lot of times a fender, etc., doesn't really work. One fall to the concrete floor and....
So looking around Amazon, to my amazement I see some for
less than $100. And also a Velleman unit for ~$140:
[ame="http://www.amazon.com/VELLEMAN-HPS140I-HANDHELD-POCKET-included/dp/B004QYOXQS/ref=sr_1_11?s=industrial&ie=UTF8&qid=1372036407&sr=1-11"]VELLEMAN HPS140I HANDHELD POCKET SCOPE 40MS/s (probe included): Amazon.com: Industrial & Scientific[/ame]
And these two that for the <$200 price look amazing, come with probes, and have fairly good reviews:
[ame]http://www.amazon.com/DSO203-72MHz-STM32-NEWEST-VERSION/dp/B006J4FZMO/ref=sr_1_3?s=industrial&ie=UTF8&qid=1372037070&sr=1-3[/ame]
This really is a 4-channel unit, for which most complaints are that it's real bandwidth is "only" 10-30 MHz.
[ame]http://www.amazon.com/SainSmart-DSO203-Portable-Oscilloscope-Bandwidth/dp/B0057M7YLE/ref=cm_cr_pr_product_top[/ame]
Lots of discussion about low-cost scopes here:
http://www.eevblog.com/forum/testgear
for example this survey of few $hundred scopes:
http://www.eevblog.com/forum/testgear/big-scope-comparison-(need-clarification)/
and check out the spreadsheet he assembled!
https://docs.google.com/spreadsheet/ccc?key=0AnnYGMOtVmrVdFZrT19PR1ZTOTFKSHFzZFBpT3hiTmc&usp=sharing