It really doesn't matter, both have their positives and negatives. A dial thermometer is more reliable, it will usually last longer depending on the manufacturer. And it's fairly water proof, so if you drop it in your boil or spill liquid all over it then it should be fine. However they are not nearly as accurate.
Digital thermometers are much more accurate but not waterproof. Nor are they as reliable, depending on the manufacturer. Additionally, some cheaper digital thermometers take a while to get a reading; they slowly count up to the correct temp.
Personally I prefer dial thermometers but that's just me.
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