For Toshiba laptops, you can look at the web site http://linux.toshiba-dme.co.jp/linux/
and the forum
to see which models are well supported under linux. In the past, Toshiba laptops with
Phoenix BIOSes had some issues with ACPI support (suspend to RAM not working, Fn+FX keys not supported, no possibility to adjust brightness of the screen) while laptops with Toshiba BIOSes were well supported by the Linux kernel.
There was an Omnibook module that allowed linux to support some of the Toshibas with Phoenix BIOS, but it was not
part of the mainline kernel and is now unmaintained.
For Dell laptops, I have only tried Latitude C600 and Latitude D630. The very old C600 used APM and had a dysfunctional trackpad. On the D630, the ACPI was fully supported, but I experienced problems with
kernel oopses at boot time or keyboard auto-repeat becoming uncontrollable.
Based on that, I would tend to recommend a Toshiba laptop, but if possible you should check before how well
ACPI works with Linux on the particular model you are thinking of buying.