There are some interesting similarities and differences, a couple of which follow. The Book of Margery Kempe and Chaucer's "Retraction" are both written in Middle English and share distinctions in common while being different from contemporary English. For example, both have a vocalized -e inflection at the end of modal verbs, like schulde, and use /y/ in words like begynnyng [beginning].
wythowtyn begynnyng how many schulde
for his grete mercy foryeve me the synne.
One particular example of how their spellings differ from contemporary English is Jhesu Crist, which is now spelled as Jesus Christ. It might be said that Chaucer has more refined spelling than Kempe. This is of interest since spelling differed within dialects and per person. For example, Chaucer's spelling for the contemporary English "souls" and "prayers" is sowles and prayers ["preye" is different in meaning: "Now preye I ..."], while Kemp's is sowlys and preyeris (Chaucer: Skeat. Chaucer Glossary)
namely of my translacions and enditynges of worldly vanitees, the whiche I revoke in my retracciouns:
I cry the mercy, Lord, for alle the sowlys that arn in peynys of purgatory ther abydyng thi mercy and the preyeris
One way "Retraction" and Margery Kempe are different is that they have different intents and therefore very different content in places. In Kempe's, she gives thanks and asks blessings for all she can think of, e.g., "for alle the sowlys that arn in peynys of purgatory ...," and says that anyone who attributes good to her is attributing good to God, not to herself:
thei that han seyd wel, I pray the, Lord, rewarde hem for that is thorw here charité and not thorw my merytys,
In Chaucer's, he also gives credit to God for anything that is liked in The Canterbury Tales,
Now preye I to hem alle ... that if ther be any thynge in it that liketh hem, that therof they thanken oure lord Jhesu Crist, of whom procedeth al wit and al goodnesse.
But he goes on to say that anything that displeases should be attributed to his unintentionalal "unkonnynge," or "unknowing" or ignorance. He then goes on to say something very different from Kempe.
Chaucer asks Christ to forgive him his sins and guilt in his translations [of French and Italian works] and his writings about worldly vanities, which he therewith retracts. He retracts and renounces some of his most valued works including Troilus and Criseyde, The Book of the Duchesse and The Canterbury Tales. He exempts from retraction his translation of the Spanish work of philosophy, Boece de Consolacione, stories of saints, and other moral writings.
enditynges [writings] of worldly vanitees, the whiche I revoke in my retracciouns: as is the book of Troilus; ... the book of the Duchesse; ... the tales of Caunterbury, thilke that sownen into synne; ...
Critics debate the voice and meaning of this retraction. While it is generally thought that it is the actual voice of Chaucer and not that of the ironic narrator of the Tales, there is room for debate: it may not be Chaucer's sincere voice; it may be the ironic narrator having one last laugh. In either event, Kempe's voice is her own and she is sincere.