I guess for me it would depend on what you want out of the testing. At age 5, she'll have less "head room" on the WPPSI, so if you think that she might be eligible for DYS, this could be a consideration. If you just want information for your use now, then this might be less of an issue.
Our experience with the WPPSI with a 5-year-old (also newly five) was that DS hit some ceilings without getting 19s. In other words, the test ran out of questions in some areas before he reached the termination criteria for them. This led to him getting 99.8 and 99.5 in verbal and performance, but was only in the high average range for processing speed. The large discrepancy in scores meant that the WPPSI gave him a full-scale IQ at a percentile in the mid-99s (99.5). We really needed help from DYS, so these scores didn't work for that, though seeing the processing speed score was really eye-opening.
When we retested on the WISC-IV, DS easily cleared the 99.9 barrier in several areas, which did give us DYS-level scores. In our case, the WPPSI ended up providing information (processing speed discrepancy) but was not that helpful because of the ceiling scores. We felt like we got more information from the WISC, where DS still got some ceiling scores, but where there was enough head room for him to show a bit more fully what he could do.
As for your concerns about being slow to answer and so on, I'd recommend mentioning those to whatever tester you do end up using. I'd also suggest looking for a tester who will talk to you about how he or she tries to establish rapport and elicit cooperation and answers from shy children.
