I hope that following will be helpful

https://www.ncbi.nlm.nih.gov/pubmed/18428673

Have you checked that they both have the same charge? The net charge will have a strong impact on how well a compound is taken up by cells. I realize the label on the vial probably says 'anionic', but in practice this simply is not always the case. Most commercially available dextran conjugates are complex mixtures of different compounds (different molecular size and charge, different labeling efficiency, in some cases even unconjugated dye molecules).

I would strongly advise to run an electrophoresis on an agarose gel of any dextran conjugate you are considering to use. A shameless plug of a blog post touching on the subject and showing an example gel is linked below.

Good luck!

http://labrigger.com/blog/2013/04/10/variability-where-you-really-dont-want-it/