which do scientist prefer dna or morphology

Hey Jeff

It really comes down to which scientist you choose and the nature of the question that is trying to be answered. Let me provide you with two examples related to arthropod phylogeny:

1. For a long-ish time, it was thought that Hexapoda (insects and their kin) and Myriapoda (millipedes, centipedes and others) formed a group called "Uniramia", to the exclusion of Chelicerata (spiders, scorpions etc...) and Crustacea (shrimps, lobsters, crabs, isopods, etc...). The main character  shared among "Uniramians" were the uniramous legs and antennae (i.e. limbs that are composed of a single element, as opposed to biramous limb which have two). As you can see, this classification is based on purely morphological characters.

    However, with the onset of molecular phylogenies and better techniques to perform them, a new result came out. Now, the phylogeny of major arthropod groups is composed of a group called Pancrustacea or Tetraconata, which considers that Hexapoda and Crustacea are actually much closer phylogenetically between them that either is to Myriapoda and Chelicerates.

    Thanks to this new way of looking at the data, an ever increasing amount of evidence has been found recently that supports the classification proposed by the molecules (Pancrustacea), rather than that favoured by strict morphologists.

2. The second example represents the other side of the coin. Chelicerata has been considered for a long time as the most basal group within arthropod phylogeny, having changed very little since the first representatives of each major group originated. Morphologically, chelicerates are quite unique and share relatively few characters with all the other arthropod classes (Myriapoda, Hexapoda and Crustacea). However, a novel hypothesis has been put forward recently that supports the idea that Chelicerata and Myriapoda form a group (named Myriochelata or Paradoxopoda). These hypotheses have in part been supported by molecular phylogenies and also some aspects of the development of neural tissues. The problem is that morphologists are very reluctant to accept such hypothesis because these groups are really very different in terms of their anatomy, but even now every now and then there is a study that appears to support this interpretation.

I hope that this rather lenghty answer helps you to have a better idea of how sometimes scientists will prefer one method or another for a number of different reasons, and that it all comes down to the particularities of the question and how to interpret the evidence available.

I agree with Javier regarding phylogeny inference - both approaches are useful in different contexts. It's also worth noting that DNA is not always available (e.g. if you are dealing with extinct organisms).

Also to interpret the question slightly differently - many biologists (including myself) are actually interested in the determination and evolution of morphology, rather than in estimating phylogenies. If you want to know how a trait will evolve it is important to think about selection on it (the way in which morphology influences fitness) and the genetic variation for the trait (i.e. arising from differences in DNA between individuals). So personally I don't prefer morphology or DNA - I use both to try and understand the way in which traits evolve witin a population (so called "microevolution").