Neither, seriously neither.
I assume this is the "4G" Slide, which at 14.4HSPA+ (*not* DC) is hardly worth that name. (The "3G" slide has a truly horrific SOC though -- totally forget that.) That's a 1.2ghz dual-core Snapdragon, but with a small WVGA screen and Gingerbread. (Nice camera for the time though.) Hard to believe this thing came out around the same time as the SGS2, which is still quite current.
The Nokia is even worse-specced (single-core Snapdragon), but the OS can't do as much so performance may be similar. It, too, is on a dead-end OS version (WP7.5), but with orders of magnitude fewer apps than the Slide.
If she's looking for a cheap T-Mo upgrade, point her to the deals on Amazon (can't you get a near-free S3 now?)... if she's going secondhand, don't go lower-end than a SGS2 (Hercules) or GSM Galaxy Nexus.